The "key" you are being prompted for is the one you created and entered in the router's security settings when you enabled WPA. If you don't recall what it was then, while connected to the router by ethernet cable, *NOT WIRELESSLY*, go back into the settings and check what it is. Some routers "mask" the key with dots or asterisks, in which case you would need to create a new key and make a note of it. You then enter this key in the PC when connecting "wirelessly" to the router.
In the PC, open a Command Prompt window ("Start > Run", type CMD and click OK) and, at the prompt, type...
...and press Enter. This will show details of all network adapters in the PC. Look for the *DEFAULT GATEWAY* address in the details of an adapter that is connected to the router - that is the router's IP address.