@Reverend Jim: It won't be much of a lock if the user doesn't have a password setup.
@OP: Reverend Jim's solution is your best bet if you want to lock the OS and not just your app. You just have to make sure the current user has a password setup, or anybody can click on the icon and "unlock" the session.
If you don't go to the OS for the lock, then anybody can bypass it by using OS shortcuts like Alt+F4 or task manager or Windows logo + E/D/R or Alt + Tab or...
It won't be much of a lock if the user doesn't have a password setup.
That's pretty much a given. The question was how to lock the screen. Just locking the app doesn't accomplish that.
Come to think of it, there is one more possible. Before I retired I used to put an app called Transparant Screen Locker on all our servers. It would lock the console but leave the display visible. It was useful in that we could keep the servers secure and not have to unlock them to check status, etc. It is available (in a free version) at http://download.cnet.com/Transparent-Screen-Lock/3000-2092_4-10153460.html