To me, the biggest drawback of XP is that one cannot do a serious PC hardware upgrade, because it will think it has been copied onto another PC.
Whereas with win2k you can just install the original CD onto a completely new machine and there is no online registration.
I believe that once software has been bought, the user has an absolute right to use it in perpetuity, on hardware which is of course his responsibility.
This is the same argument as with the dongle anti-copy devices. In 1991 I spent £10k on some FPGA design/layout software; then the dongle broke and it became useless. Luckily some Russian posted a crack for it
With XP, the solution to this situation is the full professional version, I gather, but that's very expensive. There are cracks for XP but then one cannot install SP2...
So I stick with win2000. I still run NT4 at work - it works perfectly and we don't need USB there.