Do you want to use the gEforce again in the short term? If not, remove the nVidia software for the gEforce card, shut down, and remove the card. When it restarts, hit the key (usually del, f1, esc or other) to enter the BIOS. In the 'integrated peripherals' section, there should be an option to enable the onboard video. Enable it, save changes and exit, and you should be good to roll.