CPU Shuts down by itself

I was using my computer one day when my computer screen suddenly turned totally black. Rarely, like one in eight times, my desktop will restart by itself. Other than that rare happening, it just goes out and nothing happens next. I thought it was some power-saving thing on my desktop or screen. So I tried to stop the power-saving move by pressing the keyboard or mouse. When the screen goes totally black, the screen stand power light starts to turn from green to blinking green. And the power light on my CPU stays green without blinking. In other words, the CPU stays on but I have no way to on my screen again. (Even by pressing to switch it off and switch it on again)

Pretty often, the screen goes black soon after start up for around 3 seconds and then come back. Around another 3 minutes later it goes totally black and the paragraph above this happens.

I figured it could be a virus but I need to confirm myself before posting a HiJackThis log. Also, I've checked all plugs and sockets. All are placed properly and all the way in. No worries on that.

Please assist me in this problem of mine with your kind skills. (:

Regards,
Jay