XP. XP. XP. (Does that count as three votes?)
Vista is still not up to snuff. It's not nearly as well supported by third-party hardware and software vendors, whereas XP is almost universally supported. Software that ran on Windows since '95 will often stop working in Vista. Vista may intentionally degrade hardware and software performance when watching high-definition (HD) commercial content (i.e. a DVD). Vista requires a larger portion (much larger if you go with Aero) of your video card's time, your CPU's time, and your RAM, which leaves less for other programs. There is no reason as yet to upgrade to Vista. The only real differences are in its ultra-paranoid level of security, and its DRM-enriched media playback system which often makes it actually much harder to watch HD content than on XP. I would only ever get Vista if it were cheaper than any other version, in which case, I would format the harddrive and install another OS on it as soon as I got it. But that's just me.