For firefox: Find firefox.exe (Probably at %SystemRoot%\Program Files\Mozilla Firefox\firefox.exe). Right-click on the file and select "Properties." Click the "Security" tab. Delete all the listings in the "Group or User names" box except for SYSTEM and Administrators (in my case I would be deleting Power Users, Users, and my own username). This allows access only to people in the Administrators group, which I assume this client is in.
My Computer is a bit tougher the way I'm thinking about it. Maybe take away "List Folder Contents" permissions for the C:\ Drive for User accounts? If you mean access to the applet that appears when you right-click on My Computer and select Properties, I have no idea. I get the feeling that User accounts can't mess with anything in there anyways, but I could be wrong on that.