|"I had installed a Radeon video card, and left the onboard video alone"|
What do you mean you left it alone? When you install a card, you should disable the onboard in the BIOS. There are several different settings that may need to be addressed...enable/disable, shared memory, Initial display 1st, etc. Each BIOS is a little different. You *should* have done that before you installed the ATI card (whenever that was).
Now that you're changing cards, the 1st thing you should have done was uninstall the ATI drivers. Then you should have shutdown & swapped cards. On the reboot, the new nVidia card should have been detected & you would have been prompted for the drivers. I just had a look at the nVidia site & v185.85 is the latest version available. What makes you think the onboard is being re-enabled?