[X4U] Connecting to airport but not internet

[Gail Grainger]

Hi,
    I have a friend staying with me who is using a MacBook with OS 
10.5. She can connect to our airport, but not to the Internet. When I 
did the network diagnostics, it showed it could connect to the 
airport but not the internet. I have tried comparing the settings on 
my iBook G4 running 10.4, and my iMac with 10.5, but we can't see 
what is stopping it. She uses the computer at her home and her school 
with no problem. Does anyone have any idea what else we could try?
Thanks,
Gail Grainger