when you re-installed xp, by any chance did you disconnect the internet connection? it's always a good idea to not be connected to the internet when flattening your system.
the best combo used to be (for me) antivir+blckice defender (firewall) with spybot
now, it's avast + sygate pro + spybot. i don't get virusus, but my system deos become unstable after a while but i think that's because i'm constantly messing with the os.