I am not in front of my Mac right now, but all you have to do is to delete you system preferences cache.
I had the problem once, and it drove me mad until I found the solution.
The fact that you can log in as root and have no problems is a sign that the cache files you must delete may be in your (user) library folder.
I also don´t remember if you have to log out for the trick to work. For sure, you should not delete those files with your system preferences app open.
HTH