How can I set up password protection or loggin features on a G4 every time it starts up?

Keychain does it but it only restricts access to other linked computers on the network. My problem is that, mine is the only mac in the whole company and it is constantly messed around with. So every morning, some of the extensions have been misplaced, files moved from their location and so on.

Can I protect by setting a loggin feature with restricted user access? Thanks