AirPort Sees Wireless Network but won't establish connection

Another question about my daughter's PB.

The airport card sees her wireless network but will not connect to it thus no internet connection. Self assigns an IP. the menu bar displays the 'radar' pattern with all bars ... good signal.

She has reset the router/modem and tried to manually configure. Both did not work. Her roommate (has a PC) is able to access the internet so the router/modem are good.

Has not tried to connect via ethernet, I believe. But I have asked her to try.

My line of reasoning is, if able to connect via Ethernet, then it is an airport card issue.

If unable to connect via ethernet, then it is a system problem.