You would disable the onboard video in the BIOS, and then add a video card to the PCI or the AGP slot, depending on the mainboard. Once in Windows, you'd install the drivers, previously downloaded from the manufacturer's website. These drivers should be extracted into a new folder. Then you would go into the Device Manager, choose to upgrade the current display adapter from the properties of the device, and browse to the folder where the drivers were extracted. Once the drivers are located, choose your video card, let Windows install the device, and then reboot. When once more back into Windows, go to Display Properties, and select your preferred resolution and refresh rate.
Well, if the onboard video isn't working at all, then you'll have to install the new video card first, then go into the BIOS, make the changes, and save them. There shouldn't be any conflict between the two devices while still in the BIOS.
Some mainboards also have jumpers for selecting the onboard video and audio, so take the time to check your mainboard manual for information like this.
Be sure to select which type of card will be used while in the BIOS, PCI or AGP.
By the way .. if the onboard video on a mainboard <i>did</i> suddenly just stop working, I'd want to know why. And that might involve taking the system to a local shop and having a diagnostic run on the board.