Did you do a clean reboot of windows?
If not what card did you have before, and did you clean the old drivers completely?
Every time i install some new catalyst drivers i do the following, i run 2 programs after the reboot.
1: driver sweeper
2: driver cleaner pro
then i reboot and install the new drivers again.
also what psu do you have, just to make sure it deliveres the sufficient power.