LockIt

Protecting your PC from unwanted use can be tricky, time consuming, or inconvenient. Lockit provides you with a simple, secure and fast way to do this.

Just set a password, click 'Lock' and your computer and all the applications on it are then protected from anyone else using them until the correct password is entered again.
It is ideal where constant logging off and back on again using Windows built in user system is a constant pain especially when you're the only user for that computer and you don't want to have to type in your password every time you turn on your PC.