Failure to reconfigure AirPort Express

I would really appreciate advice from the network gurus. I had a system which worked pretty well using an iMAC, Airport Extreme, one of the old AirPort expresses (b/g) and a new AirPort Express. I had some blackout spots in the house and occasionally my music would stop and restart so I thought perhaps by shifting things round a little bit I could improve matters.

My first problem is that I cannot configure the old AirPort Express. The AirPort utility finds the device, I can rename it and get a green light up until I try to get it to join the network created by by AirPort Extreme. When I try starting it up after asking it to join the network, it disappears from the airport utility which informs me that it cannot reconfigure and I should close and restart. I had no trouble reconfiguring the n AirPort Express. I've tried several resets to no effect

Can anyone help?

Follow-on questions:

Once I get the above fixed, I would like to attach another wireless router that I have (not an airport device) to the network to try and fill some of the blackspots. What is the best way of doing this without upsetting the Airport network.

Many thanks

Eugene

When u reset the old express - did it come up with the MAC address for the unit in AU? MAC will be on external label. Might be worth getting a copy of this to help with set up.

Thanks for the quick response. The Airport ID on the old express matches what comes up in AU. I'll take a look at the article.

Thanks

The article is part of a book available for purchase.

Any other suggestions ? from anyone ?

thanks