Re: 6600 GT OC problem
It is the GPU core itself that is damaged. Notice that 2D apps work fine but any application that uses the 3D functions (e.g. direct3d or opengl apps) of the card exhibit this "pixelation." I am guessing that the windows media center application is somehow 3D accelerated or else it wouldn't be doing this.
In my case I am 99.9% certain that this was caused by an overheat. If an integrated circuit overheats, you can run into all sorts of issues. All it takes is for just one of the transistors to cook just enough so that its chemical composition changes so that it is no longer conductive, and then you're screwed.
If you run the card in a relatively warm room for a long time, while running a highly GPU intensive game, even if it isn't overclocked or if you aren't using a custom cooling solution, it is possible for the thing to burn - no matter what brand it is. Theres a law of physics that applies here somewhere.
At the time I was doing this, my room was pretty warm (~95F), which meant the area around the computer was even warmer, and I was playing quake4. I had a sudden reboot, and after that the GPU just didn't work right since.
It also pretty much goes without saying that overclocking your card too much for too long will cause this, as well as improperly installing a non-stock cooling solution.
There is also the chance that the particular GPU you have was just one of the few on the assembly lines that ended up being fabricated poorly, and one or more of the transistors was defective just enough that it wouldn't last as long as it should.
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