1- Download the latest driver for your GPU from here: http://support.amd.com/us/Pages/AMDSupportHub.aspx
but DON'T RUN IT.
2- Download Driver Sweeper from here: http://www.guru3d.com/category/driversweeper/
and install it.
3- Disconnect from Internet (turn off your router) and run Driver Sweeper, select all the ATI and nVidia drivers, select analyze and after that, clean.
4- After system reboot install the driver again.