Windows 7: Videos cause power failure

As stated in the thread title, whenever I play a youtube video on my PC, it powers off. Originally I thought it was a problem with the browser or the flash plugin, but after doing some research, I found that it is caused by the video card. Something is going wrong, to confirm this, I ran a stress test with ATItool, this caused power failure, and then, in order to be absolutely sure I tried to play "Team Fortress 2" but it crashed at the menu. Initial thoughts might be that it is overheating, which I will check, but I doubt that is the problem since I have a quite well ventilated full tower case.If anyone has an idea about what is causing this, please let me know, because I really don't want to have to replace my video card.

I found the problem, the video card is overheating, because although the rest of the system is running at a cool 30 degrees Celsius, the video card is running at 98, upon investigation I found that the fan on the card is not running at all, thus allowing such overheating