First up, you didn't really explain what you wanted.
Second, my point about viewing the screen on a locked machine is still valid.
Third, how secure do you want this machine? If removing the keyboard and mouse from it (no input devices = no chance of people trying to geuss the password) is not suitable, then why not lock it in a room with a bullet proof glass panel in the door.
If only you have the key, then you can go in and do what you want to the machine, and everyone else can just look at it through the window.
Scratch that, if you destroy the key, then it should be even safer.