Locking out the system from changes

I would like to lock my system (and still be able to use it while locked) so that my teenage son cannot make changes, erase history, etc. But I would also like him to have the ability to use the system when he wants. FYI I do have netnanny installed also. Is there anyway to lock certain folders and programs from unauthorized changes? I guess this would include email as well (Outlook)

Liz