Using a USB Flash Drive as a Computer Lock- How to?

Hello everyone! I've got a question about computer privacy!

If you're anything like me, you're having to put up with your family hijacking your personal computer at odd times when you're out of the house, and, frankly, they are not internet savvy enough to understand which websites you should and should not trust to visit.

I know I could just change my user password and such, but what about this?

Is it possible to take a USB Drive and mount some kind of script or software onto it that will effectively make it a "key" to "start" your computer? If you don't have the USB or some copy of it, you have no admission to the computer's contents beyond the log in screen. If you do, fine, you can log on and use it normally.

Any ideas? I've been thinking about this a lot lately, and have tried to find some kind of software to make it possible, but the internet refuses to give me an answer. What are your suggestions?