Hi,
Try using the uninstall utility for Ati products (inside the Ati folder in Program Files)
Then Remove all files with ATI at the begginning from windows folder.
Restart
Go install the drivers again.
Anyway try finding out if You use the latest drivers. If the problem persits - go and install newer drivers - but when You are on the newest - try instaling the step back version.
Best Regards,
Piotr