I had a problem a couple of years ago where the computer would shut down by itself shortly after restarting. It turned out to be a vius.

I got instructions how to remove it from the Microsft site.

Have a look at this link it may be your problem.

www.microsoft.com/security/incident/sasser.mspx

If that fails call in a compurter specialist.
Last time I had a problem it was fixed for about
£50.00 which sounds a lot but the time u save you can be fixing cars.

Best wishes

Peter Cooper