I have a 2007 iMac and a 2011 iMac. I can install both OS X and Ubuntu (Linux) using a flash drive. Both operating systems use the modern EFI/GPT boot/partitioning method. I can not install Windows 7, 8, 8.1 or 10 using a flash drive on either model. Rather, I use a reusable RW DVD and install using the legacy BIOS/MBR boot/partition method. When holding down the alt/option key at startup, a "Windows" labeled icon indicates a BIOS/MBR install. A "EFI Boot" icon represents a EFI/GPT install. Since your Mac is a 2009 model, I would assume the same would apply to you.
If you have the iso and are using Yosemite, you can burn a DVD using the Disk Utility. Open the Disk Utility and drag the icon representing the iso file from the Finder to the Disk Utility. Highlight the iso file and click on the Burn icon located at the top of the Disk Utility window.