Enter "Safe Mode" with networking
uninstall ALL video drivers from Add/Remove programs under the control panel
Once that is done, Restart in safe mode again then go tohttp://www.guru3d.com/category/driversweeper/
Get Driver Sweeper and install it.
Now run it and select Nvidia - Display and ATI - Display if you had an ati card.
You could go delete the Nvidia and ATI(if you had ati) folders if you wish.
Restart in normal mode and try to install the latest Nvidia drivers from nvidia.com. DO NOT use the drivers from the CD that comes with the card, they are always out of date.
Hope this helps
EDIT, Beat to it by lothdk