Of all my struggles to get free internet last summer, doing what milo said, and wiping the slate clean was often the only way to get it going. When finally I got a good signal and WEP typed in carefully . . . nothing. Get rid of the list, and away it went. Happened several times.
This forum introduced me to inSSIDer 2.0 , a freebie for which I'm often very grateful. If nothing else, it will give you the lowdown on which channel to use.