The performance can be slowed by spyware and links to IE6 so first try these:
Spybot -
http://www.safer-networking.org/en/index.html Adaware
http://www.lavasoftusa.com/software/adaware/ If you are using IE then try Hijack this -
http://www.spychecker.com/program/hijackthis.html Internet sweeper pro -
http://www.ladybugsoft.com/DownloadHQ.htm Reg cleaner -
http://www.jv16.org/ I use all these with no problem, they should help speed up your PC, i also use Mozilla instead of IE6.
You must also run a firewall (other than the standard XP one) try Sygate.