Internet sharing - port problem

I am connecting an old Imac with OS 10.4 to my Airport network. In sharing preferences, I selected Internet sharing using Airport. The computer will no longer see the active signal nor connect to the internet. Now I can't edit internet sharing preferences either. Apple Support says, "If your Internet connection and your local network use the same port (built-in Ethernet, for example), investigate possible side effects before you turn on Internet sharing. In some cases (if you use a cable modem, for example) you might unintentionally affect the network settings of other ISP customers, and your ISP might terminate your service to prevent you from disrupting their network." The other two macs on the network are unaffected. Therefore, I'm not positive this is the issue. Help. Anyone. Is there a fix for this?

Does the old iMac with 10.4 have a Airport Card fitted?

Thanks for responding. The old Imac has an airport card. Before I set the preferences it saw the network (showed a dark fan).

My understanding is that if you are running 3 macs of one airport then you don't need sharing on as each mac can connect to the airport in its own right. the older airports would handle 10 computers the newer ones even more so try turning sharing off.