Restart And Shut Down Problems


Hi, I have an 17" flat panel iMac 800MHz, running on OS X 10.3.9. The problem is that I cannot log out, restart or shutdown the computer without forcing it by pushing the power button.

In the login window, the restart and shut down buttons are not active options. When I am logged in, selecting "shut down" doesn't quit applications, and selecting restart doesn't do anything either. I used Tech Tool Pro 4.0.3, ran all possible tests (on hardware, file structures, drives etc..) Everything "passed", yet the problem remains. I re-installed a new version of the system (10.3), yet the problem remains!
I searched on the Apple support database but couldn't find anything about that issue.
Can you help?