You shouldn't forget to lock your PC, even when leaving for a short while. 'Log Off' and 'Lock' are different things. This article describes how to quickly lock your computer running windows OS. Also few other useful Windows logo shortcut keys that you can use and save time.

All Windows OS (including Windows XP, Win Vista and Win7) provides a quick option to instantly 'Lock the computer' with just a simple shortcut key. The "Windows logo key+L" shortcut can lock the computer. Lock option is also provided in Windows 7 start menu.

"Lock" is different from "Log Off" because your computer and processor still runs if you lock your computer, and power is consumed same as in case of normal operation. But in Log Off, processors stops working and very nominal power is consumed.

You can also use the following keyboard shortcuts with a Microsoft Natural Keyboard or any other compatible keyboard that includes the Windows logo key and the Application key. Here Win is used to mean Windows logo key.

WinDisplay or hide the Start menu
Win+LLock the computer
Win+BREAKDisplay the System Properties dialog box
Win+DShow the desktop
Win+MMinimize all windows
Win+SHIFT+MRestore minimized windows
Win+EOpen My Computer
Win+FSearch for a file or folder
CTRL+Win+FSearch for computers
Win+F1Display Windows Help
Win+ROpen the Run dialog box
Win+UOpen Utility Manager
Application keyDisplay the Shortcut menu for the selected item

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Microsoft Support Article