I was trying to install Oracle 9i Application Server and after the file copy process was completed, during the configuration phase, I received the following error:

Oracle Database Configuration Assistant: ERROR
Invalid database domain name. The database domain name must start with an alphabetical character.

Needless to say, I never specified a database domain name, so why is it telling me that it's illegal? If it made up a database domain name on its own, why did it create an illegal name? But no, it supposedly uses the computer's name. Which does start with an alphabetical character, so what's the problem?

I checked and the computer had its name properly set up. What is more, reverse IP lookup worked properly also, so if Oracle's setup program used the machine's IP number, it should have been able to find the corresponding name with no difficulty.

Except that it didn't. Apparently, on Windows, the stupid thing requires that the name of the machine be also included in %SystemRoot%\SYSTEM32\Drivers\etc\hosts.

Incidentally, while you're at it, it's also a VERY good idea to uninstall all antivirus software and turn off the Windows Firewall before attempting to install Oracle. You can reinstall and turn these back on later, but if they're present during the installation, in my experience the installation will likely fail during one or more of the configuration steps.