Um guys. XP and 2000 are essentially the same OS. Any virus or bug that exists for XP, also exists for 2000. There's no logical reason to choose 2000 over XP anymore, except if you want the oldschool windows flag on your start menu button.

Vista works nicely, but it likes to use a lot more resources than XP, though it can still be slimmed down to about the same as XP uses, if you know what to do. Then you're essentially left with a more stable XP, and a few new features. Oh also, a few select applications will have problems starting in Vista out-of-the-box, but can usually be tweaked to work.