START > CONTROL PANEL > HARDWARE AND SOUND > DEVICE MANAGER > DISPLAY ADAPTORS

Once you have followed the above steps itll show which card you have installed, write it down mate, then i think itll be best if you do the following

START > CONTROL PANEL > PROGRAMS > UNINSTALL A PROGRAM

Once in there you should see a video driver, it should be named either ATI or AMD as they own ATI. uninstall that and anything else ATI gpu related ie control centre etc then reboot.

Now go to the AMD website which you can find here

http://support.amd.com/us/gpudownload/Pages/index.aspx

Add your info into the fields and itll bring up the correct driver for your gpu and os.

If you need any further help give me a shout.

Dave.

EDIT: Probably best to get your correct driver downloaded to your desktop before uninstalling the current one then everything wouldnt of gone massive on your desktop lol.