Download and burn this onto a CD:
http://rescuedisk.kaspersky-labs.com/rescuedisk/updatable/kav_rescue_10.iso
Boot it from the disc, and remove all of the infections.
Ur having the virus called Win32/Trojan.Shutdowner.
It shuts ur pc down every once in a while.
If this doesn't work for u,
try to google the virus itself and find other solutions.