Block computer on selected time

Program should block computer usage in selected time periods. Some window opens which asks to enter password, if not entered - does not allow to use it. When password is entered - it should be possible to set date/time periods when computer can be used. Program should work on Windows XP/Vista/7 Installation should be done as easy as possible. It should not appear under Control Panel/Task Manager and so on. If you want to uninstall it - password must be entered or other security measures taken to deny easy uninstallation. It should not be possible to kill it process using Task Manager -> Processes.