The enabling/disabling of on-board video is usually done in the BIOS of the PC. When your PC first boots you should see something on your screen like "Hit DEL to load BIOS settings". If you don't see something like that (it can go by pretty quick) then check the manual that came with your motherboard.

Once the BIOS menus are present look for a setting in there for on-board video. Set it to Enabled. Then Save and Exit. You can now try hooking up a monitor to the on-board video out and see if it works.

If it does then you can power down the PC and remove the vid card.

DISCLAIMER: If this doesn't work don't blame me! I'm not a PC expert but I play one on TV!