There’s quite a few ways to turn your pc into a motion detector, the most common seems to be using a webcam and flash.

This works well, but here are a few ways to do it differently:

This is the DIY way

Turn Your PC Into a Motion Sensing Security Device with .NET.

NB the ultrasonic sensor in above article can now be found at Parallax inc here

This guy hooked up his PIR motion sensor to a logitech mouse to simulate the mousepress and turn his monitor on.

The Go!Motion is a complete expermenters tool, so not so good for the kiosk market but interesting nonetheless. It costs $135,-

Circuitgizmos.com has a USB PIR motion detector for $44,99. It looks like it’s pretty basic and still quite DIY.

The KSL Embedded Glob-x USB PIR sensor goes for $55,- and looks most like a plug and play product. It comes with some lovely software to set the sensitivity and what actions to perform when triggered.