Installing Video Card

I have an old PC with an unknown motherboard that has onboard video. The driver for the video adapter is not available and the only setting that I can get is VGA and 16 colors. I decided to order a Matrox Millennium II 4MB PCI card. When I install the card and boot the PC the card is found and the driver is loaded. However, the Device manager shows that the card is not working because I cannot have two video adapters on my PC. Is there a way to disable the onboard video adapter so I can use the new Matrox card?