Lock Windows PC using USB
Predator locks your PC when you are away, even if your Windows session is still open. It uses a regular USB flash drive as an access control device, and works as follows: It’s easier and faster than closing your Windows session, since you do not have to retype your password when you return. The free version offers the following features:
Monitoring optionsAlarm optionsPassword optionsLanguage selectionAutoStart with WindowsCheck for updatesEnable Pause.
Predator free download
More about Predator at its download page. Also check out some more free software to lock Windows using USB.