Although I am a Mac person, I am trying to learn how to do green screening on a PC. My husband and daughter have PC's with Windows 7. I sat down at my computer and researched how to use the chroma key feature in Windows Live Movie Maker (Windows 7) and ran into a little snag. It seems that when Microsoft went from Vista to Windows 7, they changed many of the features. You can't do green screening with the new version.
Solution: Download the old Vista version on your Windows 7 machine. I found in a Movie Maker forum a blogger named Blaine who explains what to do.