The WEP key is set on your wireless router. If you configured your router with your web browser, you can view/set the key by entering the IP address of your router into your address bar. The default IP address for your router is likely printed on the underside of the router, along with the default username and password. Selecting the option to show your wireless password in Windows should also reveal your password. If you set your key using a CD that came with your router, it most likely installed a program that allows you to change the key.