Here we switched to another antivirus. Some can't do that?
FYI - I have recently seen 4 PC's whose users complained of extremely slow performance. In EVERY case, the user was an AT&T U-verse or DSL customer running Windows XP and AT&T's McAfee download. All the users were up-to-date with service pack 3.
Each system ran as though there were two anti-virus packages running simultaneously on the desktops.
After checking temp internet files, fragmentation, error checking the hard drive, and doing system file checks, and seeing no improvement, I simply removed the installed McAfee product and reinstalled it to fix the problem.