A couple of points:
I haven't used XP a lot, but from what I've seen, so far I prefer Win2K. First off, the whole Product Activation thing is something that I morally disagree with, and refuse to participate in unless I have NO OTHER CHOICE (but then again, Microsoft IS a monopoly..) Secondly, the new interface makes me feel like I'm using some version of Nickelodeon interactive TV. I am not an AOL user, and I do not need a dumbed-down cartoonish interface. I also do not need to be incessantly reminded to sign up for a Passport account that I do not need and will not use. Win2K is stable, compatible with everything that I use, and has a clean interface with relatively few bugs.
If you want to upgrade your operating system, I would strongly recommend backing up whatever data you'd like to keep, and then reformat your drive and do a clean install. Your system will thank you for it. I simply do not trust the upgrade process to put everything where it's supposed to go, and you will still have overhead from all the crap you've installed over the last year or so that you've had the previous OS installed.
-j