Predator is a software which can lock your computer with a USB Drive. It generate a security code and place it on the USB Drive to identify it as the key. If the USB drive is unplugged, the program will lock your computer then.
If someone attempts to access your PC while the USB stick is removed, a dialog is prompted and ask for the password. If the correct password is not entered within 20 seconds, an alarm will sound and access is denied. The software will log the access attempt and show you the details when you unlock your PC.
Simply plug the USB drive to unlock your PC. If you lose your USB drive, you can also use the password to unlock it.
This software has an enhanced security feature. It will change the security code in the USB drive frequently. If someone copy the image of your USB drive, it will not work as the security is already changed.
This software is free for private use.
Further information of this security tool can be found here:
http://www.montpellier-informatique.com/predator/en/
I feel that there are some really important points about this subject. It was exactly what I was looking for. Thanks very much.