Is it possible to use a MacBook as a keyboard/mouse on a Windows computer, so everything I did on the MacBook would be reflected on the Windows computer? Would this be possible perhaps in some form of a Target Display Mode?
There's no native way to do this. However, you may be able to achieve what you want through the use of software. Below are some options I can think of that you could test:
How well any of these work for you (or meet your needs) will depend on your particular requirements.
Install Synergy on both the Mac and the PC. What you are talking about is software KVM for Keyboard, Video, Mouse. You will be able to move your mouse from the laptop screen to the PC screen. When your mouse pointer is on the MacBook you can use its keyboard to type there, when it is on the PC your keyboard will type on the PC. There is also a function to share the clipboard between them, so that you can copy something from your Mac and paste onto the PC and visa-versa.
Target Disk Mode is something else, it's to use one Mac as an external hard drive for a second Mac. http://osxdaily.com/2010/04/07/how-to-boot-a-mac-in-target-disk-mode/