Make sure you don't have any other previous graphic cards installed, (Onboard doesn't apply to this)

Heres how you should install:
1. Take it out of the box, and put it into your AGP or PCI slot(Which ever is applied to you)
2. Turn the computer on and when your computer if fully booted up (some comps have "Lag" when you turn the computer on for about 10-30seconds) put in the installation cd that the card has given you.
3. Install the graphics card using the installation cd , and following its directions.
4.Most likely after you complete you gotta restart, then volia it should be working.

IF YOU DONT HAVE A Installation CD
1. go to http://www.nvidia.com/page/home
2. Go download the latest Nvidia Geforce GRAPHIC drivers and choosen the correct OS and etc etc.
3. Download and install the new driver.
4. Follow the directions given and volia it should be working.