I have a gateway desktop which a few days ago began shutting down all by itself seemingly at random. I have already checked my power settings and done a virus/spyware sweep of my computer. I doubt the computer is overheating. It seems to shut off suddenly like there is a power outage or something. Has anyone had any issues similar to this one?

Overheating usually manifests itself as the system freezing, shutting down is a sign of the power supply starting to fail. Swapping it out with a known good one is the only sure way of checking. "I've always been mad, I know I've been mad, like the most of us..."