If this is in your own home then wire a computer to the wireless router, logon to the router using the router username and password, go to the wireless settings and take a look at the key.
If this is on a Apple Airport type router then you use the Airport utility program on your notebook that is Wired to the Airport to get the key. You can downlaod the Airport utility for Windows from the Apple website. If you wire the notebook to the airport you should be able to get online and download it.
P.S. How much you want to bet there won't be a reply to this.