I take it you have fully updated windows.
The download of CSS could be a coincidence.....is it possible the GF6800GT has broken?
If you have the same problem even after a full reinstall then it points to a hardware issue.
Try reseating all the RAM modules and try running one module at once.If there is a fault in one that would identify the rogue module.
Run Memtest click here to be doubley sure about your RAM.
If you can get your hands on an alternative graphics card or try yours in a different pc that will make sure the graphics card is ok.
Go to Start>Run and type dxdiag.Check through all the tabs to see if any Directx errors show up and to make sure all your graphics hardware acceleration is working ok.