Unearth

So today I was using the my brother's computer and it would randomly.. like shut off? Well, the screen would turn off but the hard drive was supposedly "on" (Although there wasn't any of the usual sounds that the hard drive makes when it's running) but even when I tried turning the screen off it'd shut off again because it's not on. (Like you know when you turn the screen off and it says that the HD is off and it turns off again?) Well, that happened. We ran the anti-virus and Ad-Aware but it didn't find anything. Sometimes it doesn't shut off for a couple of minutes but sometimes as soon as it logs on, it shut offs. Like when I was trying to do a system restore, it'd shut off. Does this sound like a hardware problem or a virus? It seemed like it'd shut off whenever we'd try to like.. do something with the computer like restore it or go on the internet. What should I do?

Thanks in advance!