If you have another computer connected via wifi to the same wireless router, you can click on the wireless icon in the taskbar (lower right) on that computer, then right click on the name of the connection (the one that shows "Connected") and click on Properties and then on the Security tab it will show the "Network Security Key" -- click "Show characters" below that.
Another way to get to the Network Properties dialog is Start > Control Panel > Network and Internet > Network and Sharing Center and then click on the Wireless Network Connection listed beside Connections (again assuming the computer is connected via wifi) and then click on Wireless Properties and then on the Security tab it will show the "Network Security Key" -- click "Show characters" below that.
If you no longer have any computers that will connect via wifi with the router, you will have to go to the router settings and get the key there.