Password protect your screen saver

If you don't want other people using your computer when you leave the desk, just activate the password lock on your screen saver.

Right click your desktop and click Personalize.

Click Screen Saver.

In the screen saver settings window, choose a Screen Saver ...

...set after what time of inactivity should the screen saver show up and check the On resume, display logon screen option. Click OK.

In my case the screen saver activates after 5 minutes of not using the computer.

Now if some cheeky bugger wants to set an "amusing" status on your Facebook profile, he won't be able to pull the prank through. After they deactivate your screen saver, the Windows logon screen shows up. :)