Free application BtProx monitors the proximity of your computer (laptop or desktop) to your cell phone or other Bluetooth device, and automatically locks the computer when the phone goes out of range. When you walk away from your laptop with your cell phone in your pocket, for example, the computer will automatically lock--protecting its contents from prying eyes while you're not there.

In addition, you can arrange for BtProx to launch any application at the same time that it issues the lock command--so you could automatically lock your PC and turn on your laptop security system when you walk away from the machine.