In add remove programs
Delete ATI control panel
Delete ATI Display Drivers
Go to
C:\WINDOWS\inf
and remove all all entires that start with "ati " and "oem"
Run Regedit and browse to
HKEY_LOCAL_MACHINE/SOFTWARE
remove all ATI entires.
In regedit again browse to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
remove all ATI entires once more.
Reboot. Install new display driver, after that reboot then install Control Panel.
Your system should be cleaned of all previous driver now.