In General: Restart your PC, during the bootup screen (right after the monitor turns on), hit the button that takes you to your BIOS. It may be the "Del" button or F8. Usually there is a message that says something like, "To enter the configuration menu, hit...". Once in the BIOS, using the keyboard for navigation, you will need to hunt around for the video configuration until you find the On-Board Video item. Disable On Board Video. Save Changes & Exit. Make sure your monitor is hooked up to your video card, not the on-board video.
If you can't get into the BIOS, it might be good to look up your particular motherboard on the web to see what the procedure is.