Ok, I think I have the solution. This worked for me a couple of years back when I had this same problem. Aparently in the BIOS there is a settings that changes the default video device. If for some reason this got changed to your onboard video instead of your graphics card it can cause problems. To fix it, simply unplug the power, and leave the video card in. Then look for the CMOS batter (small coin shaped battery on motherboard) and remove it. Let the battery sit out for about 2 - 4 minuets to allow for the bios to be reset (this will not harm your pc). Then put the battery back into the case, and restart the computer. It should now recognize the video card as the default device.