Uninstall your old driver reverting to VGA mode. When it asks to reboot click cancel. Shut the system down , crack open the case. Install the card and make sure to attach any extra power cables needed (if needed). attach the VGA (or hdmi) cable) Reboot. When it detects new hardware and wants to look for drivers either select "have disc" and instal from there, or close it and insert the disc and let it autorun. Reboot , and you should be good to go.
The biggest thing is , if you have a consumer PC (i.e. Dell, HP, ect), is to make sure that you have enough power supply for the card you want to install. Check with your MFR for the specs, or ask on some of the many HW/SW forums that are out there.