Airport connection problem

Hi guys,
Looks like I might need your help again.
Main computer is a 2005 1.6 single processor G5 tower running Leopard.
Connection to internet is via Airport Express base station and cable modem.

My son has an older G3 Imac he uses for homework. I recently installed an Airport card so he can access his teacher's website. When I tried to connect to the Airport network, for some reason it won't connect automatically. When I type in the network name, it asks for the password and then says password is incorrect. It isn't of course. Any ideas on why it is doing this?
Thanks

 

Okay, I got a little further toward fixing the problem, but it still won't connect.
I reset the Airport Extreme to factory settings. I then created a new network with a new password. Restarted the cable modem and got a connection on the main computer. I then went back to my son's computer and joined the new newtwork. It seems like it starts to connect when I get a red flag window stating that the ip address is in use by something else. Here is the exact message:

IP 68.107.136.247 in use by 0:d:93:83:e4:1

Does anyone know what this means?

 

Forgot to add, it sees the network and the signal strength is 100%. Some sort of IP conflict, though. Won't connect.