You'll have to check your BIOS settings when you boot the PC up. As far as I know, most motherboards will disable the onboard one if they detect some other video card plugged in.

I'd guess you either want to tell it to not disable the onboard controller, or use the onboard as the primary graphics controller so leaving Windows to detect and do the rest for the other card after booting, but having not done this myself before, I can't say whether it'd work.