Airport on computer does not see any wifi networks

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Airport worked fine until recently and worked fine when I installed new OS a while back. I can see no wifi networks including my Airport Express. My other Mini sees it and others as well as my MacBook so it seems to be an issue with that computer.
Sometimes I can log into my network by choosing Join Other Network but sometimes it doesn't work like now. Earlier today I logged in in this method and then it dropped out and I haven't been able to connect again. Any suggestions as to what the problem is?
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Replay

Yes, I had thought of that too. Just wanted to avoid buying another card without being sure of the problem.
I've now tried setting up two routers to the 5GHz band, and sure enough, they both work and their wifis are picked by the macmini. All the other wifi networks I see around my house seem to be 54g.
So it seems that only the b/g part of the macmini's wifi card is broken, while the n part is working properly. Do you know if this is possible or if there is a configuration in the macmini that is forcing it to only use the 802.11N?
Thanks for your help anyway!