Have you removed the old drivers first.
Have you downloaded the full package with the new control panel. click here
If you un-install the Windows drivers and ATI control panel in the add/remove section of the control panel.
Do not re-boot until both have been removed.
(You can get the Cat un-installer from click here to remove all traces of the drivers.)
After you have un-installed both re-boot, after Windows starts do not let Windows install the drivers click cancel.
You then need you click on the downloaded .exe to install the new drivers and control panel. Re-boot and re-set your screen resolution.