If you're a little worried about someone using your computer, and want to catch them in the act, Mouse Lock is a simple web app that snaps a picture when someone moves the mouse.
Mouse Lock has you sign in with your Google account, and then you select a PIN number. You start Mouse Lock by clicking a small arrow in your browser, and walk away. When the mouse moves again, Mouse Lock starts a countdown. If the PIN isn't entered, it snaps a picture with your webcam and emails it to your Gmail account. It doesn't actually lock down your computer in any way, but it does catch the person in the act. You have a lot of options for catching people, like using Windows Task Scheduler or software like Prey, but Mouse Lock is a simple, browser solution that doesn't require any real set up. If you can't lock your screen completely, this is a way to at least catch the culprit.