Not sure which "fix" was the cause, but I did multiple Windows Update runs on a Windows XP PC a week ago that hasn't been the same since...
Monday, May 05, 2003
XP System Slowdowns Caused by Microsoft Patch
XP System Slowdowns Caused by Microsoft Patch "Reports are coming in from all over regarding XP system slowdowns caused by Microsoft's latest buffer-overrun patch. Microsoft allegedly is at work on a remedy to the problems created by its own security fix."