Try booting up without a GPU card incase it has onboard graphics chip. If it does then uninstall the original card's driver. You have only two to think of. Nvidea and Ati.
Once you have uninstalled the original driver for your gpu then log off and shut down. Disconnect from' mains' then insert new GPU card. Boot up and you should see ' new hardware' pop-up.
Ignore Windows installation wizard and use the CD that came with your GPU card to install.