exactly are you using ?
1) Uninstall the device driver
(Graphic card) in the ''device managers'' panel from the configuration panel and restart your computer.
Windows 7 will probably install a default driver, let it install before doing anything more. (if you are experimented with computers, I suggest you try using a software to clean the graphics cards drivers)
2) Download the latest drivers at World Leader in Visual Computing Technologies | NVIDIA
Select the appropriate computers settings and device to download the proper driver. (on the download section of the nvidia update you will be asked to put in theses informations)
3) Reboot your computer and tell us what it did.
Tell me if you need more details on the process. This first step will ensure that it is not a simple software corruption.