There are some initial reports of the McAfee VirusScan update 5985 causing computers to crash. I have already seen this on three systems, all running Windows XP SP3.
I would highly recommend disabling auto-update until the bad update is repaired by Network Associates.
UPDATE: If you have been impacted by this, there is a KB article here to help you resolve the issue.
UPDATE #2: There is a full fix for the issue here. It's not a very simple fix, however I have tested it and it works.