First of all, sorry for my English. Let's see if someone can help me.
I just bought a new graphich card, an NVIDIA GeForce GTX 670. I removed the previous one, plugged in the new one, started the PC and EVERYTHING WAS RUNNING JUST FINE. I installed the new drivers without any problems and decided to try the new graphic card with a game.
So I started playing the new Tomb Rider. To test my new graphic card I set the video settings to Max, and the game was running just fine. But suddenly, in a in-game cinematic, the PC shut down and restarted itself.
Thing is, the PC boots until the "Windows" screen (you know, black with the Windows 7 logo), then the screen goes black, the motherboard emits a short beep (half a second or so), and the PC restarts itself. And then again, the same thing: windows screen, black, beep.
So I tried to change back to the new graphic card and the PC boots just fine. So I'm gessing it's a problem with the new graphic card, but as I have already told you it worked for a time (maybe 2 or 3 hours).