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 install your new graphics card, making sure it’s firmly connected into the socket and connected to your monitor.
Boot the PC and enter the Setup menu to access the BIOS. Go to the 'Advanced Features’ page and select to 'Disable Onboard VGA’. The wording and location may vary according to your BIOS. Save changes to the BIOS and exit. PC will reboot, Windows should detect new graphics card and ask you to install its drivers.