Matt Ryan of The Frugal Geek blog on Lockergnome shows you how to lock your Mac OS X Screen using a password protected screen saver. From the Desktop & Screen Saver settings in Mac OS X, be sure you have turned on a screen saver. You then configure a hot spot on your screen to activate the screen saver when you move the cursor to that location. Configure the password for your Mac OS X user account. From the Security System Preferences page, check the box to require your password be entered when screen saver is activated.