I had this problem last month. My PC was misbehaving and will mysteriously 'die' sometimes while using it midway (usually while web browsing - it doesn't matter which browser). All I would get is just a blank black screens and nothing moves. I did get the blue-screen-of-death but not often.
Once it halted while booting - it didn't go beyond the WinXP logo. And at another time, it suddenly rebooted while I was trying to login in.