Insert your new graphics card into the AGP port
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.