Why does my PC shut down while I'm using it?

Brad built a computer running Windows and now it just shuts down while he's still using it. Leo says it sounds like a bad power supply. Brad replaced it, though. Leo says that's the downside of building his own machine -- he'll have to hunt down the problem himself. Leo suspects that since the power supply didn't fix it, the motherboard is probably the culprit. It could have been from a power surge. He should try replacing the battery for the BIOS and see if that resets it. If that doesn't work, it could be the motherboard. The next thing to address would be the CPU.