You may have to enter the system bios and disable the onboard video before you can use the. You will still uninstall the current video drivers, but you'll have to check to see if you need to disable onboard video first.
To enter BIOS, you tap something like the DEL, F2, ESC, or some other key when a mesage that says something like "Press DEL to enter setup..." All motherboards are different, and if you don't see the message, just try different keys until you find it. Once you are in the BIOS use the arrow keys to look for something like "Onboard video." If that option exists, set it to "Disable" or something similar if you need to. Then you exit BIOS, saving the settings, and turn off the computer. Install the video card, etc.
If you don't see a setting for onboard video, your mobo may automatically detect a video card and use that instead of the onboard graphics.