Spyware / AdWare - [Pending] automatic shutdown posted in the Security & Safety forums; please help, PC automatically shutsdown more often. I can't even finish scanning my files....
The last time that sort of thing happened to me, it was a virus or some other infection, but I had ME back then. If it's not the heat issue, have you tried starting in Safe Mode? To get there, just tap the F8 key as you get to the splash screen, (the one right before the windows screen,)
What are you using to scan and did you notice other problems before the computer started shuting itself off?