The NVidia 6800 does support open GL so there should be no issues there. Sounds to me that you have either got corrupt video drivers or worse (as has been suggested), a dieing video card. I have recently had to change my 6600GT to an ATI X1300Pro. The 6600 died after rendering a pretty hefty MR Scene.

So my advise is as follows:

Go to Add/Remove Programs and deinstall the software for the card (do not use the card manufacturers software always use NVidias latest software from their site). Once done do not reboot quite yet. Go into Device Manager and remove your video card (Right Click on My Computer and Choose Device Manager), highlight the card and hit Delete (not backspace) and say yes to any prompts or confirmations. Do not reboot yet, if you have not got the latest drivers go to www.nvidia.com and download them.

Now reboot and once your up again you should get a nice low res screen (usually running at 800x600). Install the NVidia drivers and allow a reboot once done reset your resolution to the one you want and away you go. If this does not work then I am afraid you may have a dieing card.