Ahem.
I wasn't going to say anything, but I just can't help myself....
Win98SE and Win2K were the last incarnations of Microsoft being worried about competition.
WinME and WinXP are both just Pretty, dumbed down interfaces of the 2 operating systems; In respect to Win2K vs WinXP, Win2k is more stable, has a lower official failure rate, and runs the same applications faster.
WinXP also has a lot of security flaws built in that just shouldn't be there, since they almost had to be by design.
Most pro organizations that require security and stability are completely ignoring XP, waiting for Windows 2003 (which, by the way, is pretty nice).