First, go to
ATI's website and download the latest Catalyst driver for your machine.
Go into Add/Remove Programs and remove anything "ATI". Go into the Device Manager (right click My Computer, click properties, click Hardware tab, click Device Manager) and look under "Display Adaptors". Delete any ATI hardware listed there. Restart your computer.
After the restart, Windows may detect your video card again and attempt to install a driver for it, cancel this. Install the ATI Catalyst driver you downloaded.