Are you behind a router or firewall?
I would think about blocking all the unneeded ports on the firewall or router.
Also turning off all the unneeded services in "services.msc". look at
www.blackviper.com at his "xp services" configuration....he tells how to turn off unneeded services which frees up some resources but ALSO closes up some security holes. I pretty much use the "safe" config except for a few points such as I leave on the restore service etc...and he is assuming the comp is by itself but if its sharing files you probably need to leave on "server" and maybe other file sharing type stuff...each service has a description with it to tell what it does and also its "dependencies" (some services need other services to be on for them to run)
Look here for some info about closing some ports
http://www.blackviper.com/AskBV/tech10.htm
Also look here
www.grc.com and do their "shields up" scan for "common ports" and see what it tells you.
Do all the windows updates.
JP