I'm one of the evil pirates who bought 1 copy of W2K and installed it on 4 or 5 machines. I frequently cull old parts that are laying around and put together new machines, installing that same copy of W2K.
With XP I'd be screwed. I'd need to buy separate copies for each machine and beg MS for permission each time I changed something . . .
XP - no way. I will switch to Linux when W2K becomes obsolete.
As far as I've been able to research, XP is nothing more than Win2K with snazzy new GUI face paint and some video oriented CDRW multimedia apps thrown in for good measure. The initial complaint against 2K was poor gaming performance, but it appears to be no coincidence that service pack 2 for Win2K brought it up to almost identical 3D and graphical frames per second performance with XP. It looks like the only thing that XP buys you that Win2K doesn't is the intrusive anti-pirate registration feature that monitors hardware upgrades, notifies Redmond, and shuts down the OS if the new configuration looks too much like a new computer.