XP is stable like everybody says but all told it is just Big Brother Bloatware. It is full of spyware and is a control freak. Unless you run exactly what Micro$oft tells you to run or use (especially with hardware) it won't run that smoothly at all.
It is just another attempt of M$ trying to totally dominate the computer market and push every else (that isn't in cohorts with M$) out.
XP doesn't do anything that 2000 won't do, alot of sites that review software actually say 2000 is slightly faster and 2000 plays just as many games if not more (although some need tweaking) than XP