How to lock a laptop without affecting image being sent to TV?

How to lock a laptop without affecting image being sent to TV?

I have a laptop connected to two monitors at work, but I need to lock the laptop so that no-one can use it while a powerpoint presentation is running through the TV screens.
The operating system is Vista. Is there any way to make the laptop tamper proof and still have the TVs showing the powerpoint presentation from the laptop??
Keep it simple please folks, I'm a rookie :-)