Could be a temperature problem, the processor is over heating and shutting off. If you can try running real Temp.
http://www.techpowerup.com/realtemp/
If it blue screened or performed a memory dump I would say its a mobo, ram or HDD failure.
It could very well be a virus thats in the bios of the mobo. The only solution is to flash the bios. This can be risky since if something goes wrong it wrecks the mobo so you may need to take it to someone else but we can cross that bridge if we come to it.
For now check your temps and report back. If it isn't temperatures then we can look into other causes.