Did you repair your permissions after you installed/ upgraded to Panther? By default, installations of OSX are far from perfect. Repairing permissions is a good place to start.
If the problem persists, shut your machine down, then restart while holding down the Apple+S keys, which reboots into the single user mode. Type fsck -y as instructed at the prompt, and let it run, repairing any problems. Repeat until it says 'No problems found', then type reboot. Your machine will restart.