A fairly popular article I posted a couple years back is on setting up iTunes for multiple users on Windows. I just updated that article so it shows how to do it in XP and in Vista, and I fleshed out the steps so they're easier to understand and follow. (I just had to do this on a new laptop we bought, so I figured I'd offer it up to folks and see if I could spread the joy.)
If you're into the multi-user iTunes thing, check it out.