MondoWin.com

PREDATOR locks your PC when you are away, even if your Windows session is still opened.
It uses a regular USB flash drive as an access control device, and works as follows:

you insert the USB drive
you run PREDATOR (autostart with Windows is possible)
you do your work...
when you're away from your PC, you simply remove the USB drive:
once it is removed, the keyboard and mouse are disabled and the screen darkens
when you return back to your PC, you put the USB flash drive in place:
keyboard and mouse are immediately released, and the display is restored.
It's easier and faster than closing your Windows session, since you do not have to retype your password when you return.