
This works for windows 7 only , this will not work for Windows XP but keep reading to find out more.
Under "Security" tab click "Show characters" to see your network key.
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Another way is to use netsh in command prompt. You don't necessarily need to be connected to the network. This step can be used to recover password for notworks that are not in range. Only requirement is that you have been connected successfully at least once in past.
Go to "Start - Run" than type "cmd" and hit "Enter" to open command prompt window.
You can do same with shortcut "Win+R" type "cmd" and press "Enter". Use following commands:

netsh wlan show profiles
This command will show all profiles - Wireless networks that you connected in past.
netsh wlan show profile "your network" key=clear
This will show more detailed profile information. Under security settings you will find key content - that is your network key. Type your network name without quotes.