There are two elements to disabling an onboard graphics card and sometimes you need to do both before the system will work properly. First, get your system back to how it was without the new card installed. Now go to the Device Manager in Windows, right-click the entry for the driver for your onboard graphics card and select Disable. Apply the changes and shut down your PC.
Now insert your new graphics card into the AGP port, making sure it’s firmly connected into the socket and hook up your monitor.
Boot the PC and enter the Setup menu to access the BIOS typically by pressing [F1] or [Del]. Go to the 'Advanced Features’ page and select to 'Disable Onboard VGA’. The wording and location may vary according to your BIOS, but you should be able to find it. Press [F10] to save changes to the BIOS and exit. Your PC should now boot recognising your new graphics card and prompt you to install the new drivers for it in Windows.