This is most likely to be a driver issue, AGP can be sensitive to hardware changes, so I'd suggest you do this:
Download and install either Driversweeper or Driver Cleaner.
Download the latest drivers for your card (they'll be quite old, Nvidia stopped supporting AGP some years ago).
Uninstall all the existing Nvidia drivers.
Uninstall all of the motherboard drivers.
Restart into Safe Mode and from there use the driver cleaner program to remove any remnants of the old software.
Now install the drivers but note that some AGP boards are sensitive to the order in which drivers are installed, so read up and install in the order given in the manual. If you have lost it, most manufacturers websites have online manuals.