Is it a recent CCC?
In the old one (when i bought mine one year ago) you had to right click on something.
It was really hard to find.
In the newer one it's much more easy to find.
Check all the tabs, in one you will find "enable overscan" and check the box.
Now you can modify the overscan bar.
I can't tell you exactly where it is because i've sold my 5830 and don't have CCC any more.
When i reinstalled windows 7 recently, i had to do the trick again, thus it is not a problem solved by amd and it is worth trying.