http://news.cnet.com/8301-1009_3-20003074-83.html
If you haven't updated your McAfee software yet today, WAIT! It seems a programming error is causing the software to classify a critical Windows file as one that is malicious, causing Windows XP to get stuck in a rebooting cycle.
Work around instructions are here:
http://vil.nai.com/vil/5958_false.htm