You can lock down a PC with the Windows key + L, pulling up the logon screen...
or using the Predator (http://www.montpellier-informatique.com/predator/en/) utility, you can install a key on a USB drive that has to be inserted into a computer (or multiple) for it to be used. Pulling the USB will disable the mouse and keyboard plus darken the screen. The USB has to be reinserted to unlock the computer.
Thanks to scrubjay from Tent Dwellers. O0