Wolfmoon: can't be any worse than the 98 se i'm running. XP and 2000 are both NT-based, so neither would crash a lot. The main advantage XP has over 2000 is that it is very customizable.
ME is the most unstable of them all, that's why XP came out to "combine 2000 and ME."
Zachquiel: well I think they're all basically the same gameplay; go around, if you see someone, kill. Occasionally you'll meet a teammate, lots of weird stuff.
Of the ones you listed, i've only played Halo and Counterstrike. The main difference in FPS is the speed. Some games are fast, like UT and Halo, while others require other things, like counterstrike does. Counterstrike is more of a careful-play. I guess that would be a difference.