Click on the Start button and then select the control panel option from the start menu. A window will open.
Select the Network and Internet option from this window. Another window will open.
Select the Network and Sharing Center option. Yet another window will open.
Select the Manage Wireless Networks option ( on the left ). A list of networks will display.
Select the one for which you want to know network security key. A window will open.
Check on the check box which says Show Characters. You should be able to see the network security key for the network you have selected.