On-board video and a video card?

I have a Biostar M6VLQ motherboard, which comes with an on-board Trident Blade 3D video.
When I put my PCI nVidia GeForce 2 MX200 in, the on-board video doesn’t display anything and when I plug the monitor into the GeForce all I get are vertical lines.
When I take the card out the on-board video works fine.
I tested the GeForce in a different computer and it works fine.