i'm running XP on a 500mhz Celeron, it's just BRUTAL how slow it is.
my game server was actually a good comparison. while the Athlon box was being formatted, i had to move the server onto the Celeron. the lag was so incredibly bad when 8 people were on, everybody would have 500 ping which is just terrible. i set the server to have realtime priority so my dad is on the thing trying to get on the internet but he notices that his mouse isn't moving "god damn cock sucker" he yells "get over here, this thing is so cock suckin slow" because it was pulling even the resources for the mouse towards the game server.
about 2 or 3 weeks prior to that, the Celeron had Linux Mandrake 8.0 on it (it had to be formatted since it was completely screwed). i put the exact same game server on the Linux machine and it was able to handle about 12 or 13 people before it started to lag...pings would still be under 200 though which isn't that bad.
i think i'll be with my Linux computer again soon though, WinXP won't let me register and the crack doesn't work so Linux might be on its way.