Software To Lock Computer?

im having a bit of an issue, im looking for something very specific, some kinda software where i can just initiate a lock on my computer, see i have 3 kids and my sons usually enter my room and totally destroy my desktop, along with opening many instances of games, even sending out email haha, doing things they are not suppose to be doing.

Anyway, i figured hey ill just put a really short screen saver password on my computer, well they still play with it, trying to use or get into it, they end up shutting it off via the button in the log in screen area after u move the mouse to deactivate the screen saver.

Im wondering if one of you guys know out there something i can run to lock my computer up while im not using it, but still keep running my programs/servers in the background, similar to a screen saver password only with no option to shut my computer off, i can disable the screen saver password in light of some other software where i could maybe just double click on something and then enter a password when i want to unlock it

if anyone has or knows a good piece of software they trust they can suggest to me im more then willing to give your suggestions a try, a quick Google search yields some results however i have trust issues, "what if i lock myself out of the computer permanently..etc" and would rather have some experienced users , plus maybe it will serve others looking for the same thing in the future