try to remove you VGA card, and turn on your PC again...
couple months ago, my old rig start to turn off by itself. when i turn it on again, it could only last for like 5 or 10 minutes and it start to turn off again... then i removed my vga card, and it doesn't happen again. it turns out that my vga card fan is dead so its overheating.... thanks god the rest of the hardware is ok...