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.
Finally, if you did not set a key at all, your router is likely using a default key. Like the username and password, it is almost certainly printed on the underside of the router.
Warning: WEP is very insecure. Anyone with even a modicum of computer literacy can run a program to crack it in a matter of seconds. Unless you are absolutely sure that you need it and your router and devices do not support WPA or WPA2, you should not use WEP encryption.