But, Computer Commando, XP (and XP Pro) have some extra additions which take some elements from the 9x series so that both branches (NT and 9x) could be merged to form one solid but friendly OS.
MS tried (and failed) to do this with 2000, which is why ME came about.
However, I must say that I have made a mistake. According to EA, the Sims 2 is in fact compatible with 2000. My bad.