McAfee sent out a bad update yesterday that wiped out a Windows System file (svchost.exe) and rendered Windows very unhappy. The system I was dealing with initially lost network connectivity and many programs stopped running properly. I ran a Repair installation for XP to fix the issue.
They now supply a tool to fix the problem: http://vil.nai.com/vil/5958_false.htm
You figure that sort of thing would show up in testing. XP SP2 and XP SP3 were affected.