airport and network issues - help!

I have an imac running OSX 10.4 that uses airport to connect to the interent via a dlink base station. It is my partner's computer and two days ago his internet stopped working and says "error joining network". I have taken the computer upstairs and plugged it into the modem directly and - it works! My macbook pro also works, wirelessly as before without any network or base station problems. The imac. even after plugging it in and then unplugging right at the base station, still can not connect to the station even though its name comes up in network preferences etc.

I found that this can happen if my partner happenstancely accepted an airport driver update in which case I can use the method in this thread http://www.mac-forums.com/forums/switcher-hangout/52388-reverting-back-older-driver-version.html to fix it. However, my software update doesn't work because of "a network problem" even though the ethernet is plugged in (and I am online through it right now). When I go to the support page http://support.apple.com/downloads/ to download the old driver manually it just spins and spins and spins....

Any ideas would be much appreciated!

It's working now!

After the thrid time unplugging the base station while the imac was shut-down, I restarted the computer. It still could not find the network and had the same error.

I then used the network setup assistant and it works!

Not sure what the problem was , but thanks to anyone who looked at this.