can't you take the agp card out again...........then boot up.......uninstall the drivers for the onboard. then uninstall the onboard hardware in device manager, and disable the beast. (right click--disable)
remove any recently installed nvidia drivers in add/remove programmes
reboot in safe mode with the new card installed and run driver cleanner to remove the last traces of the onboard drivers. and any nvidia grapphics drivers remnants.
then reboot and install the new card drivers and reboot to complete installation.