Have you recently been inside the machine? If so, you might want to check for a loose screw.
Software issues causing that problem include certain AV packages, incorrectly installed video drivers, IRQ conflicts with other devices. Is your video running all by itself or is it sharing IRQ space with another PCI device?
Not all of them like sharing.
Next thing: Is this happening at a specific, repeatable event? Are you trying to do something that requires DirectX/OpenGL support when this happens, or are you just wiggling the mouse around when POOF! you're rebooting?
Next - check your screen saver...if it's doing this when the screen saver kicks in, there's the chance that you've got a corrupted one (I saw this just the other day).
Finally - scanned for bugs, worms, and virii lately?