I had this happin before and it was just a matter of getting the slider back in time to make it stable agian, im useing the card right now, but thanks for your help.
Try this then. Enable hidden folders; then load your User-Profile Folder. Go to Application Data; search for Ati Catalyst Control Center. Delete the profile XML Document. That folder contains all of the values for Catalyst Control Center to remember to boot with. Without it; it should default to the original application install values.
That should allow you to boot normal again; without the Overclock. Let me know if that helps. As I said though; ensure that Catalyst Control Center and Drivers are removed as well. Then delete the folder; then reinstall both.