While looking at the permissions of my laptop and PowerMac, they both have different permissions set for the Group. On my PM, it is Group unknown, on the TIbook it is my shortname as the Group. I am able to change the Access permissions on each Mac.
Can you change the Group to your shortname, and then set Access and others to No Access ?
The only difference on my PM is the fact that I am sharing all the hard drives on it. What happens if you create another new Admin account?
Have you installed any 3rd party software, like SharePoints ?