Overheating could cause this kind of problem, but if you're running your components at the stock speeds it shouldnt be happening. To rule this out you can download a tool called motherboard monitor, which will monitor the temperatures on a few of your system components.
http://www.pcworld.com/downloads/file_d … RSS,00.aspYou should be able to monitor the temps on your graphics card via the forceware control panel, just right click on your desktop, go to nvidia display, and click on whatever is listed there. There should be a temperature tab.
If its not actually caused by overheating, my next guess would be a serious malfunction in windows somewhere or a faulty component. If you're not even getting a bluescreen, it means that windows, or your hardware is failing in a major way. The rest of your system specs would help with diagnoses, you should refer to the stickied thread about posting your computer problems in this section of the forums.