Have you tried booting into safe mode? and if so do you get the same result? I would suspect a failing hard drive or a bad memory stick. Are your graphics card drivers up to date?
I would start by trying safe mode, and then if it works I would assume it is driver related, Otherwise you could always remove your graphics card and use onboard display and see if the problem persists, or uninstall the driver you currently have and use the standard vga graphics driver.. I bet it is either a ram stick or the hard drive though, I would test the hard drive first using Western digital data lifeguard diagnostics and then test the ram with Memtest86