Portable Mono/.NET
I was playing around with Mono (an open-source .NET implementation) and decided to try it out as a portable application development suite. I installed it on my computer, copied the files to my USB drive, and tried to hook it up with PSPad (which is already portable). I can now compile C# code from PSPad without any dependancies on .NET being installed.
Compiler Settings
Here are the compiler settings for PSPad I used...
Compiler: drive:pathto\Mono\bin\mcs.bat
Parameters: "%FILE%"