My home PC keeps restaring itself as soon as XP has booted - i literally get 5 seconds with my desktop before the PC restarts - every fucking time!. WHY IS DOING THIS AND HOW DO I REMEDY IT???!?!?!?!

Having perused pc-type forums, it would appear i may have contracted a virus or something etc. but i want an idiots guide to sorting it. I can get on in Safe Mode if that's of any use.

Advise. Thankings.