You can usually find it when you login to your router. I didn't this like 20 minutes ago.
To login to your router, enter your IP into the address bar and login if it asks you to (a lot of routers have
admin as the user and
admin as the pass).
This works for Windows 7, so I don't know if it's the same for XP. To get the IP, run
ipconfig from the command line. Mines called Default Gateway and its 192.168.2.1 (yours should be similar).
My WEP key was under the Home Network section when I logged into my router and it was labeled as 128-bit key (or something similar).