I just cleaned a virus off a laptop that did this. It was the Sasser Worm.
This worm may cause LSASS.EXE to stop responding, which forces the operating
system to shut down after 60 seconds. If your computer starts to shut down,
follow these steps to abort any system shutdown that may be in progress.
1.
On the taskbar at the bottom of your screen, click Start, and then
click Run.
2.
Type: cmd and then click OK.
3.
At the command prompt, type: shutdown.exe -a and then press ENTER.
If this stops the shutdown cycle, go to
www.antivirus.com and run a free
online scan of your system to find out what virus you have. If it's the
Sasser Worm, go to this site to clean it:
http://www.microsoft.com/security/incident/sasser.mspx
If it's another virus, go to Google and search for the name of that virus
along with the word removal
Hope this helps
C.E. Jones