I've sorted out numerous friends whose computers were crawling so slowly they thought there was something wrong with their (relatively new) well-specced machines.
Apart from removing one spyware and two viruses (out of a couple of dozen machines), most of the fixes consisted of removing McAfee and/or Symantec firewalls and anti-virus, and installing ZoneAlarm (if they didn't have a router with a NAT firewall), and AVG free plus the usual Spyware and Adware protection stuff.
It seems that McAfee and Symantec are like a very large handbrake locked firmly ON. Maybe it's something in the default configuration, I don't know.
I usually also upgrade them to Service Pack 2 of their Win XP version (if not already there), and switch off all Windows updates.
I'm sure purist Microsoft experts and McAfee/Symantec users will be horrified, but in every case that approach has sorted the problems.