Make sure in the Device Manager that the onboard, or integrated, graphics card is "Disabled," do not uninstall it.  Check in the "Add or Remove Programs" (XP) or "Programs and Features" (Vista/Windows 7) program if there are any drivers you can uninstall for that onboard, or integrated, graphics card.

In some cases, you have to either disable or select which graphics card is the primary display in the motherboard's BIOS.  Common sections are "Primary Video Adapter", "Primary Graphics Adapter", "Init. Display First".  For further assistance, you can always consult your motherboard's manual or website.