Wireless network interference?

Worked with a friend who has an Apple AirPort Extreme wireless base 
station on the corner of the 2nd floor of his home. A PowerBook about 
12 feet away gets a strong signal, and another PowerBook 50 feed 
away, downstairs, gets marginal, sometimes zero signal, but the one 2 
feet from the BS gets nothing. The AirPort card utility (this is OS 
9) in this one computer shows only the neighbor's network name.

I took this iBook home to further analyze it and it immediately it 
can pick up a network half a block away.

This really sounds like wireless interference, either from the 
neighbor's network or another non-802.11 source.

How should I go about troubleshooting this? I know that I can change 
channels of devices, but when and how? If I put the base station on 
another channel, do the cards automatically detect it and select the 
appropriate channel when you join the network? Or is there more to it?

As far as playing detective, what are the best tools and methods for 
determining the source of the interference?

Any suggestions and comments would be very welcome.

Thanks,
Dave
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