software kill switch

I've encrypted my disk, but I need a way to prevent somebody from stealing the computer while the encryption key is in RAM.

Basically I need to write a program that writes zeroes to the memory and then cuts power to all devices. The problem is, I don't know exactly how to accomplish this.

Advice preferred in C++ or assembly, but anything would be helpful.

You can make your own thermite from powdered aluminum, iron oxide (rust), and some putty to hold it together. The tricky part is making the ignition system, you need a thin strip of magnesium and some way to fire that up.