Same thing happened to me. It's the "LoveSan" worm. Here's a blurb from AP:
"Symantec Corp., F-Secure Corp. and other anti-virus companies have free tools for removing the worm. All users, whether their computers were infected or not, should also obtain Microsoft's fix by going to http://windowsupdate.microsoft.com.%mark-on
(They should also update any anti-virus or firewall products they have by visiting the vendors' Web sites."
This happened to me when i came here last night, so the worm got in when i went on-line. I manually cut power to the computer the second time it said it was going to shut down, and haven't had any problems since then. I'm going to go to Norton's site to look for the tool to remove it, as i ran a McAffee scan, and it reported it had found no viruses.