Simplicity
Explorer
I like Vista except for a couple of issues. Lots of hardware just doesn't work well with it. My printer needs to be rebooted after a paper jam. My monitor stopped working after a nVidia driver update. At this point though, most hardware is going to have Vista as a primary platform, so it shouldn't be as much of an issue.
Lots of people don't like UAC (the security prompts). They're wrong. UAC is necessary, although it's perhaps not implemented as securely as it should have been. XP boxes turn to malware-covered slag after too much web browsing. Vista boxes are much more resistant.
Vista does include some REALLY nice features. Network mapping is sweet (sure, you could add that to XP too, but it's slightly more work to do).
So I'd say go for Vista for now.
Lots of people don't like UAC (the security prompts). They're wrong. UAC is necessary, although it's perhaps not implemented as securely as it should have been. XP boxes turn to malware-covered slag after too much web browsing. Vista boxes are much more resistant.
Vista does include some REALLY nice features. Network mapping is sweet (sure, you could add that to XP too, but it's slightly more work to do).
So I'd say go for Vista for now.