Welcome to the DH forums matylda...
Run the Mobility Modder program, available here:
http://www.driverheaven.net/patje/ (follow the instructions on the page!)
on the latest ATI Catalyst 5.9 driver, available here:
http://www.driverheavendownloads.net/ (click on the ATI logo then choose the third or fourth file from the top depending upon which control panel you want to run, the Catalyst Control Panel - CCC, or Classic Contol Panel - classic)
Be sure to completely uninstall your previous driver before installing the new driver, and spend some time reading through the Mobility Radeon Drivers forum here if this is new to you...