MRT Lock is very easy to use. First, you need to choose a combination of keys that will unlock the computer whenever you want to. The default combination is CTRL-ALT-L, but you can change it if you want.
When you'll be gone from your computer for a few minutes, just open this program and click the 'Lock' button. Your computer will instantly be locked, and no one will be able to use it (unless you've taught them the secret combination).
If you use it frequently, you can configure MRT Lock to start up alongside Windows.
As you can see, this is a simple program that will keep anyone from using your computer when you're not around.