BTProximity is an application for Windows Vista and Windows 7 that can detect when your bluetooth-enabled phone has come into range, and unlock your computer.
When the device moves out of range, the computer is automatically locked. This isn't a fake lock window, the desktop is actually locked the same as if you pressed Win+L.
BTProximity is donationware, if you like it, please donate!
Download (Windows Vista/7 only – x86 and x64)
Notes
- The device needs to be paired with Windows first, then it will show up in the devices list.
- Be sure to install the unlock component if you’d like to automatically unlock.
- Be sure to set the credentials to unlock.
- Setting the check interval too high may lead to poor battery life.
- “Faster Unlock” will unlock as soon as the device has been seen, without an additional check.
- Domain logon is not supported, only a local user account may be authenticated.
(Source)