When you are using a computer in office or any public place where others can access it, you can quickly lock your computer by pressing Win+L when you need to move away from it temporarily. In case you keep forgetting to lock your PC, you can use BtProx.
BtProx is a small Windows application that takes advantage of Bluetooth’s basic ability to detect Bluetooth devices to automatically lock your Windows computer whenever you walk away from it.
To use it, you need a Bluetooth enabled cell phone or any suitable device and pair it to your computer. BtProx runs on the system tray and continually checks to make sure your phone is within range. If you walk away from your computer and outside the range of the Bluetooth device, BtProx will activate your screensaver to lock your screen or even run the program of your choice.
One disadvantage of this application is that Bluetooth needs to be kept turned on at all times which can significantly drain your cell phone battery. BtProx is a freeware and open source and it works with both Windows XP and Vista.