If you're really worried about program compatibility, go with Win2k. XP is a big hassle if you upgrade your hardware often, and can give problems if the drivers aren't "Microsoft Certified" to work with WinXP. The only advantage of XP over 2k is that XP comes with built-in eye-candy, zipping software, and burning software. Otherwise they're the same.
Oh yeah, XP is also slightly less of a memory hog if you turn all of the effects off.