If you enabled 128 bit WEP in the router, you had to create a key........
Linksys creates a 20 digit hexadecimal key - you can make one up or let it generate one by using a passphrase. If you use the passphrase, it creates 4 keys, write down Key 1 and use that.
Have you read your router manual at all? It's on the CD that came with the router or you can download it from Linksys.