Just before you leave the house, turn off the computer and open it up, now disconnect the power button wire(s) from the motherboard (It says "PWR SW" or someting like that and runs from the front of the computer to the edge of the motherboard). It takes about 2 seconds to remove and is totally effective at preventing a computer-illiterate from using the system. It's safe, too. Just remember to plug it back in when you're done.
__________________
|