I have Vista. Bryce 5.5 and PSP8 both run on it.

Many installers give you the choice of installing for everyone or just the user logged in. If you choose everyone in vista it puts parts of programs in a program data folder instead of just programs. Some programs like blender won't run when split like this.

If you tweak any programs it's a nightmare. Blender or other programs that use python or java scripts are almost impossible to edit. The system sees additions as malicious code and automatically locks them. You have to go in manually and edit properties.

Games in vista have to be loaded into a games folder or the desktop to run.
Games that run Quicktime 6.3 won't run on Quicktime 7 and you can't dig it out once it's on your machine. If you ever visit the web your browser will scream for quicktime 7 the first time you hit a movie file.

Looks like they took a step back from this crazyness with windows7. If you get to make the choice I'd pick windows 7.