The simplest way to find the WEP key for a router is to go to the properties page for the Internet connection on a computer that is wirelessly connected. The WEP key can also be found by connecting a computer to the router by Ethernet cable.
To find a router's WEP key on a computer that is already connected, open the Start menu and click on Control Panel. Select Network and Internet and then Network and Sharing Center. Under Network and Sharing Center, select Manage Wireless Networks on the left. Right-click the name of the router and select Properties. In the Properties window, select the Security tab, then click the check box that says Show Characters. The dots in the box next to Network Security Key should turn into letters and numbers.
To find the WEP key by connecting to the router, plug one end of an Ethernet cable into the router and the other end into the appropriate slot on the computer. Open a web browser and type in "192.168.0.1". This is the default address for most Netgear, Linksys and D-Link routers. Consult the manual for other routers. Hit Enter and a login prompt should appear. This does not work for machines not connected to the router with an Ethernet cord. For the username, type in "admin" and put in the password selected when the router was set up. If the password wasn't changed, leave the password field blank or consult the instruction manual. Click on the Wireless or Wireless Settings link to find the WEP key. Depending on the router model, the WEP key may be under a sub-menu with a title like Security.