AirPort Status menu connects to wrong (non-existent) network

I have a strange issue I'm hoping someone might be able to help with. Each time I start up my iMac, my AirPort Status menu lists a single network to join - one that was automatically created when I first got my iMac a few days ago and set up the connection to my AEBS. Since then, I have changed the name of my computer and created a new network. Now, the only network listed in my AirPort Status menu no longer exists, and my new network doesn't show up at all. I can find the new network by opening Network Settings and clicking on the 'Network name' dropdown box - the correct network is listed there. At that point, the old network name disappears completely, and doesn't reappear until the computer is restarted, and it is again selected as the default network. I can't find a way to delete this old network properly, or at least tell my iMac which is the default network to connect to.

I have tried deleting the AirPort Base Station Agent from my account Login, reinstalling the AEBS software and adding the Base Station Agent back to the Login items as suggested in another thread I found, but no luck there. I have also reset my AEBS to the factory settings, but before I had even set it up, the AirPort Status menu was again listing the wrong network. I have also tried removing the AirPort from the Network Preferences and adding it back in, but it still picks up the old, non-existent network.

Please help!