BtProx is a small Windows application working both under Windows XP and Vista. I never tested it under Windows 2000, but there's no reason why it should not work. It locks your computer when you go away from it. How is it done.
It locks on one of your Bluetooth devices which are paired to the the machine. When you walk away from the computer the device is disconnected and the computer is locked after timeout. It is possible to run a single application together with the lock. This application may be your favorite script dismounting network drives, erasing those dirty files from the desktop or anything else. The program sits in Windows tray showing its current status with tray icon and tooltip baloons.

Installation

1. Just download the binary package (the one without "src" at the end of the file and place it somewhere convient, for example "C:\Program Files". Run it from there.
2. Pair your device to the computer, if you use this device already with the computer, this step may be skippeed.
3. Run the program and select your device using "..." button next to "Used device" label
4. Set the timeout after which the computer will be locked
5. Click "Start" button
6. The icon of the program turns orange as soon as the device is found
7. When the device disappears and the icon turns red, few minutes after Windows will be locked. In order to abort locking it, right-click the icon in system tray and choos "Abort workstation lock"

Screenshots

Main window of the program:

This is how the program looks in system tray:

Download

Grab the program from here: BtProx download page
Most chances you will need only "btprox.zip" file. Only if you want to hack the source you will need the second file on download page.