Here is a way to lock your system ,and take away the key along with you, so that no one can enter your system,even with your password. But except with your key...
I don't mean you to buy a good lock at the shop. Just your U.S.B. can act as the key. But you need to install a program PREDATOR. you can get it from...
http://www.montpellier-informatique.com/predator/en/index.php?n=Main.DownloadFree
Just download it, install it... while installing it will ask for the U.S.B. drive... at that time insert you insert your U.S.B. drive in its port... Thats all, after completing the installation, remove the U.S.B... Then when ever you need to open window, just insert the U.S.B. drive to the port... The window will run the predator and take the key from the U.S.B....
you could ask... even win key+L will lock the system.. But if some one knew your password.. then they can easily enter into your system... but now with out U.S.B. even you cannot enter into the windowsssss....