BleepingComputer.com: pc abruptly shuts

hey techies,there a real serious problemmo,whenever i try to play a game on my computer,or try to install some heavy software or any software,my f@#$ing pc shuta off itself.But still i see num lock lights and the green light(the one we hav on the cabinet beside the red one) on.Then i hav to hold the start button(on cabinet) untill the green light turns off to switch off the whole system. i again start it manually by using power button.I even scanned the whole hd for virus but found nothing.plz help me coz it is really annoying me and makes me really angry.
I am using windows xp sp2.

EDIT: Moved to a more appropriate forum - MG

one thing more,i thought it was a virus problem so i tried to format my c drive but again same thing happened,after the initialization of win xp setup(where we hav to chose whether we want to install a fresh copy or repair the present one) pc shut ed itself.please dont advise me to format my entire hd.

Sounds like a CPU overheating proble3m.

Could be the heatsink/fan is dirty preventing proper cooling o, a possible fan failure, or maybe both.

What brand/model is it ?

my pc is assembled one,
mercury mainboard(don't know the model no.)