Windows 8 is only supported up to Catalyst 14.4 WHQL or Catalyst 14.6 Beta, so you will have to use an older driver.
I presume you are connected via HDMI connection? What is the make and model of your display?
I'd like you to try a system clean up and a fresh installation of Catalyst 14.4 which should resolve the issue.
1. Run the Windows self check tool to check scan and automatically repair any corrupt files found in Windows. There is a guide on how to do it here. What you need to do is run an elevated command prompt (elevated means run it as administrator) and type SFC /SCANNOW. This will generate a statement indicating if errors were found the Operating system and if they were fixable.
3. Update Windows. Install all available critical, recommended and optional updates. Often Netframework and Visual C are included in the recommended or optional updates Windows provides. Windows should always be fully up to date before installing our drivers!
4. Use DDU to sweep the system of AMD drivers in safe mode.
5. Install Catalyst 14.4 WHQL once the above is completed and check to see if the issue is resolved.
Once you have the driver installed, please navigate to Catalyst Control Center > My Digital Flat Panels and adjust the underscan/overscan slider to remove the black bars.