Aplane, a few basic things here.
When you install a new video card, there are a few things you must do in order to avoid a conflict.
I have installed a PCI card twice now with no problems.
Here's what I do.
I uninstall the current video drivers for my onboard video.
You can find this in "add/remove programs".
I then disable the onboard video. I believe that in some cases you can do this right from the device manager.
I shut the computer down and reboot it. I am looking to see that my video is disabled and drivers are gone. The screen should look very ugly and big. (poor resolution).
I shut the computer off again, UNPLUG IT, open it up (be careful not to cause any static sparks etc., ground yourself to the case), and install the new PCI card.
Start the computer.
Now, this is important...follow the installation instructions for your particular card to a "T".
The only exception being that you are better off to download the latest drivers for the card from the product website.
Install the latest drivers and make sure that you have the latest DirectX as well. (click on run from the start menu and type in dxdiag, this will allow you to see what version of directX you currently have).
Follow the set up instructions for your card. You will likely need to set resolution and refresh rates etc.
Now, hopefully I haven't messed up here and hopefully if anyone spots something bad that I just said they will jump in and correct me.
All that being said, I can give no guarantee that this will work for you. Only that it has worked for me.
A quick last point about drivers. If you do not completely remove the old drivers, you may have some issues when you install the new drivers.
Cheers, Merry Xmas and hope this helps.
RTR