Mac set-up window appears 3 days after upgrade-everything was fine 'til now

Question

I posted a question recently in the iMac (CRT) forum and linked to it here (regarding disappearing finder/desktop on my father-in-law's computer after an OS 10.3 upgrade via DVDs purchased from Other World Computing). I solved the problem (or so I thought) by purchasing a different set of Panther CDs (from applerescue.com) and installing them.
Unfortunately, he can't get into his old desktop and files again. But the problem is different than the one that presented itself on that previous upgrade. At least after that one, he had access to his old files, etc.
The installation (Archive and Install) was done on Thursday. Several reboots were successful that day. I left the upgrade (from 10.3) to 10.3.9 for another time. My father-in-law started up successfully that night, Friday, and Saturday (though he ran into a problem with a printer that was not recognized - easily repaired, had that been the only problem). This morning, Sunday, he started up and was unhappily presented with the OS X welcome screen. I ran over to his house this afternoon and was unable to bypass the screen by any method I knew of (as I said, his computer behaved normally prior to today --- booted right up into his account as before the upgrade for the past three days). I started to enter information into the unavoidable fields, but aborted when I realized that I might not have access to his old accounts (and files, which we had backed up, of course).
What could possibly be causing the welcome window to show up? I've only seen it in two circumstances: 1) Starting up a new computer; 2) When doing an erase and install.
While waiting to see whether anyone has a clue as to what has happened, I will connect my computer to his and see whether all his old files are intact.
In case it helps, here is the older item regarding this computer posted in the iMac G3 forum:
http://discussions.apple.com/thread.jspa?threadID=759902&tstart=0
Thanks for your insight (as opposed to iSight).
Leonard
G5 iMac (iSight)   Mac OS X (10.4.7)   1.5 Gb RAM

Answer 1

The old iMac is now working fine, so I've marked the question as answered, even though I still don't know why the problem occurred or how, exactly, it got fixed.
Here's the latest, for anyone interested in the outcome:
When we last looked in on our father-in-law's hapless computer, it was displaying the welcome screen/set-up window after the Archive and Install of 10.3 from a new set of (supposedly) universal install disks. It was behaving like a new computer that had never been used before (or like a computer whose OS was installed after a complete wiping of the disk). An examination via our own G5 iMac (with the CRT iMac in Target Disk Mode) showed that all the files and accounts were intact, yet the computer was still insisting that we set up an account so we could use it.
Concerned that we might lose (easy) access to the old main account - even though we had backed up everything, I elected to reinstall the OS hoping to avoid erasing the disk. When I had done this a few days ago (using the new disks to install 10.3 over the 10.3.9 version that was working but not showing the desktop/finder), I did an Archive and Install, since (of course) I couldn't install an earlier version on top of a later version.
This time, I had the Upgrade option available. I have no idea why, since I was installing the same version of the software as was in the machine already. (That may be normal, however. I don't know, nor does it matter particularly.) I chose that option, rather than Archive and Install hoping the iMac would rediscover its accounts.
The installation went smoothly (though it took lots longer, since I forgot to de-select all the printer drivers and other language support). However, I was presented with the welcome screen yet again. A quick check, again via the G5 with the CRT iMac in Target Disk Mode, showed that all the files and accounts were still there.
I had nothing to lose, so I set up a new account. Once I had a desktop, I went to System Preferences/Accounts, and saw that both older accounts were still there (marked as Administrator accounts as before). I changed the login preferences, selecting the old main user account for automatic login.
I restarted, and the old desktop appeared. All the files were intact and accessible, so I updated to 10.3.9 with the matching Java update.
Hopefully that's the end of it. The previous strange behavior is still a mystery, though.
As to the disks, I understand the nature of machine specific disks and the problems they can cause, even though those eMac DVDs worked fine with my old retired PowerBook. Those disks were sold as UNIVERSAL install disks (or I wouldn't have bought them) by Other World Computing - I wonder how they can make that claim. Those disks also installed 10.3.4 on the iMac (which I then updated to 10.3.9), but the Finder did not work, and I couldn't get a desktop, as I said in an earlier message. Why they worked on the PowerBook, i don't know.
This latest set of Panther disks (3 CDs) were sold by applerescue.com as universal install disks also. They are version 10.3 only (not 10.3.x) and are not marked for any particular machine. However, they are white, not black like the illustration that Don and others have linked to as being the retail version of the OS. Are they the problem? Who knows? And where did they come from if they aren't the retail version or a machine specific version?
Sooner or later, I'd like to figure all of this out, but for now we have a working computer.
(Thanks again to Don Morgan.)
Leonard
G5 iMac (iSight)   Mac OS X (10.4.7)   1.5 Gb RAM