Which drivers did you install? When you get a new graphics card it usually comes with a drivers CD and that's where you should be installing from if in doubt.
The other option is to go to
http://support.amd.com/us/gpudownload/Pages/index.aspx
then click your OS, your card (I would guess 4600 series), and download from there.
When you say the computer doesn't recognize the card exists, what do you mean? When you go in device manager (control panel (classic view), device manager), what do you see under display adapters? If it shows another card, right click it and pick uninstall.
The other option is to go to
http://support.amd.com/us/gpudownload/Pages/index.aspx
then click your OS, your card (I would guess 4600 series), and download from there.
When you say the computer doesn't recognize the card exists, what do you mean? When you go in device manager (control panel (classic view), device manager), what do you see under display adapters? If it shows another card, right click it and pick uninstall.