i know these may be really obvious, but you havnt mentioned if you have tried anything.

it could be a few things:
1. the power supply may be playing up on you

2. as said by most others, it could be a CPU heat issue, does the shutdown usually happen when the system is under load, or idle, or totally randomly?

3. it *may* be one of those shutdown related viruses, but not likely due to the nature of it


have you checked the system logs for anything abnormal or out of place?
if your system doesnt have temp monitoring software u can use in windows, u may be able to start your system, get into the BIOS settings, where you will probably have some temp readouts. Let the system ru nfor a half hour and see if the temps get abnormally high

hope this may be of some help