You may want to try going into Add/Remove Programs and uninstalling your existing Nvidia Drivers, download the newest drivers from the Nvidia Website and reinstall.
I've noticed when installing updated drivers over exsiting Nvidia drivers, sometimes I've had problems very similar to what you are experiencing. *Update*
After reading your other post http://www.bleepingcomputer.com/forums/t/271810/re-computer-blankig-out/
it is still possible that you have a driver issue.
You may also have a Video card issue with the video card overheating or, you may also have an issue of not enough power meaning you need a bigger power supply ( i.e. 350w instead of 250w )
Hope this helps,
Edited by techextreme, 16 November 2009 - 07:27 AM.