You must have some kind of driver installed, otherwise nothing would be working. I'm guessing Windows rolled back to your previous version when you removed 10.6.
Just to be on the safe side, grab a utility called DriverSweeper. http://www.guru3d.com/category/driversweeper/
Remove your current driver, reboot in safe mode, run DriverSweeper to clean out the remnants of you old drivers, and then reboot and install the new version.
If your issue persists, post on the ATi websites. They need to know about it.