Instructions
Turn on any computer connected to the network that you want to connect your Xbox 360 to.
Click "Start."
Open "Control Panel."
Select the icon, "Network and Sharing Center."
Choose "Manage Wireless Networks." Find the name of the correct WPA2 network. Right-click the name and click "Properties." Select the "Security" button at the top of the window and check the "Show Characters" option. The exact WPA2 key of the network will be displayed in the box on screen.