UPDATE
======

An improved procedure for installing Windows 7 can be found at **[Installing Windows 7 Pro 64 bit without DVD or Flash Drive][1]**

Note:
-----
The updated steps, for installing Windows 8.1 using the procedure given below, can be found [here][2].
**********************************

My hardware and software differs from yours. I will first document how I tested my answer and then try to adapt the answer to your needs. Since this is a fairly long answer, the adaption will be given as a separate answer.

My computer is an iMac (20-inch Mid 2007). The version of OS X is 10.10.5. I will be installing Windows 7 Professional SP1 64 Bit.
I have only one internal drive. It is a 1 TB hard disk using the Apple hybrid GUID partitioning scheme. Hybrid means Windows will see the drive as MBR partitioned, while OS X views the drive as GUID partitioned. I will be following Apple's recommendations and installing Window using the legacy BIOS. **I also have a working internal optical SuperDrive which is not used to install Windows.**

(Hint: For a better view, click on an image or open an image in a new window.)

 1. Aquire the Boot Camp Support Software (the drivers). The software
    can either be downloaded using the Boot Camp Assistant, downloaded
    directly from the Web or copied off a OS X installation DVD. See
    [System requirements to install Windows on your Mac via Boot
    Camp][3] for details. This procedure does **not** require the use of
    a flash drive. *If your Boot Camp Support Software is stored on a
    flash drive, then follow the special instructions given in certain
    step that follow.*
 2. Make sure your firmware is up to date. See [EFI and SMC firmware
    updates for Intel-based Macs][4].
 3. Download and install a free copy of [VirtualBox][5] from Oracle. I
    used VirtualBox, Version 5.0.0 r101573, Copyright © 2015.
 4. Inside your "Documents" folder, create a folder named "VirtualBox".
    This is where you will keep the files created in the rest of the
    steps.
 5. Create a MS-DOS (FAT) formatted partition to install Windows. You
    can use the Bootcamp Assistant, the Disk Utility, or commands
    (`distutil`, `gpt` and/or `fdisk`) entered in a Terminal window.
    Other third party tools, such as [`gdisk`][6], can also be used.
    Give the label for this partition the name "BOOTCAMP".
 6. Determine the disk and partition numbers of the partition named
    "BOOTCAMP". To do this, highlight the partition in the Disk Utility
    application and click the "Info" button. A popup window should
    appear. The values shown will be used in the next step.

    [![][7]][7]
    
    The information, in this pop up window, shows the disk and partition
    numbers are 0 and 4, respectively. If your numbers are different,
    you will need to substitute your numbers in the rest of the steps.
    **If your partition number is greater than 4, then you can not install Windows on this partition.** (*Technically, this may not be
    true, but it is beyond the scope of this answer to explain how to do
    so.*)
    
    [![][8]][8]
    
    Do **not** close the popup window.

 7. Open the Terminal application and enter the following commands in
    the Terminal window. This will set variables to be used in later
    commands. In the next command, replace `α` with the "Disk Number"
    shown in the Disk Utility popup window. The usual value for `α` is
    `0`.
    
        DISK0=/dev/diskα
    
    In the next command, replace `β` with the "Partition Number" shown
    in the Disk Utility popup window. The usual value for `β` is `4`. 	
    	
        PARTITION=β
    
    The next command sets the variable `PARTID` to the disk identifier of
    the partition. This should be the same as the "Disk Identifier"
    shown in the Disk Utility popup window. The usual value for `PARTID`
    is `disk0s4`.
    
    	PARTID="$DISK0"s"$PARTITION"
    
    Note: the letter `s` is lower case.
    
    Close the Disk Utility popup window.

 8. With the Disk Utility application still open, select "File→New→Blank
    Disk Image..." from the menu bar. In the popup window, enter the
    values shown below. Note: "Where:" should be the "VirtualBox" folder
    created in Step 4. *If the Boot Camp Support Software is stored on a
    flash drive, you can use the default "Size:" of "100 MB".*
    
    Save As: bcssoftware  
    Tags:  
    Where: VirtualBox  
    Name: BCSSOFTWARE  
    Size: Custom... (1.5 GB)  
    Format: MS-DOS (FAT)  
    Encryption: none  
    Partitions: Single partition -  Master Boot Record Partition Map  
    Image Format: read/write disk image  
    
    The popup window should appear as shown below.
    
    [![][9]][9]
    
    Click the "Create" button.

 9. Determine the disk number of the partition named "BCSSOFTWARE". To
    do this, highlight the partition in the Disk Utility application and
    click the "Info" button. A popup window should appear. The values
    shown will be used in the next step.
    
    [![][10]][10]
    
    The information, in this pop up window, shows the disk number is 1.
    If your number is different, you will need to substitute your number
    in the rest of the steps.
    
    [![][11]][11]
    
    Do **not** close the popup window.

 10. Using the same Terminal window, enter the following to set the
     variable `DISK1`. Replace `γ` with the "Disk Number" shown in the
     Disk Utility popup window. The usual value for `γ` is `1`.

        DISK1=/dev/diskγ

     **Quit the Disk Utility.**

 11. Copy the Boot Camp Support Software to to the partition labeled
     "BCSSOFTWARE". I used the Finder application to do this. *You can
     skip this step, if this software is already stored on a flash
     drive.*
 12. Mark your partition as active in the MBR by entering the following
     in the same Terminal window. The input for the interactive command
     `fdisk` is taken from the variable `INPUT`. If a password  is asked
     for, enter it.
           
        INPUT=$(printf  "f  $PARTITION\nw\ny\nq")
        sudo  fdisk  -e  $DISK0  <<<"$INPUT"
     
     Note: To display the value for the variable `INPUT`, use the 
     command `echo "$INPUT"`.
     
 13. Allow VirtualBox read/write access to your physical partition by
     entering the following commands in the same Terminal window. If a
     password is asked for, enter it.
     
        diskutil  unmount  $PARTID 
        sudo  chmod  go+rw  $PARTID
     
     This will allow VirtualBox to install Windows on your physical
     partition. **Note: This access will only last until OS X is
     rebooted.**
 14. Create the files that map the virtual partition and disk to the
     physical partition and disk image. In the same Terminal window,
     enter the following commands. If a password is asked for, enter it.
                      
        cd  ~/documents/virtualbox
        sudo  vboxmanage  internalcommands  createrawvmdk  -filename  $PWD/bootcamp.vmdk  -rawdisk  $DISK0  -partitions  $PARTITION
        sudo  chown  $USER  bootcamp*.vmdk
        diskutil unmountDisk $DISK1
        vboxmanage  internalcommands  createrawvmdk  -filename  $PWD/bcssoftware.vmdk  -rawdisk  $DISK1

 15. Open the VirtualBox application and click on icon above the New
     label. Enter or select the values shown below, then click the
     "Continue" button.
     
     [![][12]][12]
     
     Use the default settings  except for the hard drive. Choose the
     "Use an existing virtual hard  drive file" button. Navigate to the
     VirtualBox folder created in  step 4. Open the "bootcamp.vmdk"
     file. Your window should appear similar to what is shown below.
     
     [![][13]][13]
     
     Click the "Create" button.

 16. Enter  the following command in the same Terminal window.
 
        diskutil  unmountDisk  $DISK1
     
     Return to the VirtualBox application's "Oracle VM Virtual Manager"
     window and click on the  icon above the "Settings" label. Next,
     click on the icon above the "Storage" label. Right click on
     "Controller: SATA" and select "Add  Hard Disk". Choose existing
     disk by opening the file "bcssoftware.vmdk" stored in the folder
     created in step 4. The attachment "bcssoftware.vmdk" should appear
     as shown below.
     
     [![a16][14]][14]
     
     ***It has been reported by other users that the "Use Host I/O Cache" box needs to be checked on if VirtualBox boot fails with error `VERR_NOT_SUPPORTED`.***

 17. Highlight the Empty CD/DVD and select "Choose a virtual CD/DVD 
     file..." to attach your Windows installation iso  file. (Hint: Look
     for the CD/DVD icons)
     
     [![a17][15]][15]
     
     Click OK to close the  window. Your "Oracle VM Virtual Manager"
     window should appear similar to what is shown below.    
     
     [![a18][16]][16]

 18. Before clicking the icon above the "Start" label to begin installing
     Windows, enter the following command in the same Terminal window.
     
        diskutil unmountDisk $DISK1

 19. When you reach the window shown below, select "Custom (advanced)". 

     [![][17]][17]
     
     In the next window, click on "Drive options (advanced)".
     
     [![][18]][18]
     
     Select the "BOOTCAMP" partition and click on "Format". The error
     message shown below will disappear. Click on "Next" to continue
     installation.
     
     [![][19]][19]

 20. The rest the virtual machine installation will be repeated again
     after booting Windows on the physical machine. Use the provided
     answers when the following windows first appear. When asked a
     second time, you can provide your own answers.
     
     When the following window first appears, I would enter the Product
     Key and uncheck the "Automatically active Windows when I'm online"
     box.
     
     [![][20]][20]
     
     To be safe, I would not let any updates download or install until
     after the Boot Camp Support Software (the drivers) is installed.
     Therefore, I would click on "Ask me later" in the window shown
     below.
     
     [![][21]][21]
     
     VirtualBox creates a virtual Public network for your virtual
     machine. Therefore I would click on "Public network" in the window
     shown below.
     
     [![][22]][22]

 21. Once the installation has completed on the virtual machine, you
     need complete the following instructions to make a copy the MBR
     bootstrap code. From the Start Menu, select "Computer" to open a
     window containing the hard disk drives and devices with removable 
     storage. Determine the drive letters for  the hard disk drive 
     labeled "BCSSOFTWARE" and the CD (optical)   drive. The image below
     shows the drive letters on my computer are  `D:` and `G:`, 
     respectively. Your drive letters may differ from mine.

     [![][23]][23]
     
     Close the window and open a Command  Prompt window run as an
     administrator. Select "Start Menu→All  Programs→Accessories",
     then right click on "Command Prompt" and  select "Run as
     administrator", as shown in the image below.
     
     [![][24]][24]
     
     In the Command Prompt  window, enter the command shown below. If 
     necessary, substitute your drive letters for `G:` and `D:`. 
       
        G:\BOOT\BOOTSECT  /NT60  D:  /MBR
     
     The results of running this command on my computer are shown below.
     
     
     [![][25]][25]
     
     Close the Command Prompt window.

 22. Copy the Boot Camp Support Software from the partition labeled
     "BCSSOFTWARE" to the "C:\Users\Public\Public Documents" folder. I
     used the Windows  Explorer to do this. *You can skip this step, if
     this software is already stored on a flash drive.*

 23. You need to prepare Windows to run on your physical Mac. Open the
     Windows  Explorer and navigate to the "C:\Windows\System32\sysprep"
     folder. Open the "sysprep" application. Select "Enter System
     Out-of-Box Experience (OOBE)", "Generalize" and "Shutdown" as shown
     below. Click OK and wait for the virtual machine to shutdown. 
     
     [![][26]][26]

 24. Once the virtual machine has shutdown, quit VirtualBox. Enter the 
     following in the same Terminal window. These commands will update 
     the MBR bootstrap code, set the Windows partition identifier to 7 
     in the MBR and then mount the partition. The input for the 
     interactive command `fdisk` is taken  from the variable `INPUT`. If
     a password is asked for, enter it.
     
        sudo  fdisk  -u  -y  -f  $DISK1  $DISK0
        INPUT=$(printf  "s  $PARTITION\n7\nw\ny\nq")
        sudo  fdisk  -e   $DISK0  <<<"$INPUT"
        diskutil  mount  $PARTID

 25. Close all other applications. Open System Preferences and select
     "Startup Disk". Highlight the "Untitled Windows" icon and click the
     "Restart" button.

     [![][27]][27]
     
     If something goes wrong, you will need to hold down the "option" key
     at startup to return to OS X. Hopefully, nothing will go wrong and
     Windows will start.

 26. When Windows finishing installing, install the Boot Camp Support  
     Software (the drivers). This software is either in the
     "C:\Users\Public\Public Documents" folder, or on a flash drive.

COMMENTS
--------

On OS X El Capitan (10.11) the System Integrity Protection does not allow editing the MBR with `fdisk`, so disable SIP [following these instructions](http://apple.stackexchange.com/a/208481/19237) for the duration of the install.

I ended up having to create two user accounts. If you only want one account, you have two options. First, delete one of the accounts. Second, disable one of the accounts so it will not appear. The former can be accomplished from the Control Panel, the latter requires entering the command `control  userpasswords2` in a Command Prompt window. For example, below is the window were one can enable or disable the rpj account. If you need help let me know.

[![][28]][28]


  [1]: http://apple.stackexchange.com/questions/225387/no-bootable-device-usb-2-0-macbook-pro-mid-2014/225521#225521
  [2]: http://apple.stackexchange.com/questions/189335/any-solution-to-bootcamp-windows-8-1-with-macbook-pro-15-2010/189398#189398
  [3]: https://support.apple.com/en-us/HT204048
  [4]: https://support.apple.com/en-us/HT201518
  [5]: https://www.virtualbox.org
  [6]: http://www.rodsbooks.com/gdisk/
  [7]: https://i.sstatic.net/EC8ZW.png
  [8]: https://i.sstatic.net/k02eZ.png
  [9]: https://i.sstatic.net/XnPPJ.png
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  [11]: https://i.sstatic.net/ZgAPn.png
  [12]: https://i.sstatic.net/wPeZx.png
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  [15]: https://i.sstatic.net/qDttV.png
  [16]: https://i.sstatic.net/pjb5J.jpg
  [17]: https://i.sstatic.net/ikXcW.png
  [18]: https://i.sstatic.net/oIZOC.png
  [19]: https://i.sstatic.net/dKeEf.png
  [20]: https://i.sstatic.net/Z6Dlv.png
  [21]: https://i.sstatic.net/P6vHe.png
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  [24]: https://i.sstatic.net/6LAqy.jpg
  [25]: https://i.sstatic.net/7PPoU.jpg
  [26]: https://i.sstatic.net/m2HXN.png
  [27]: https://i.sstatic.net/4732N.png
  [28]: https://i.sstatic.net/Q0O2O.png