Over the past hour I got to thinking and realized that I had not given all the info a post of this sort should have, plus, I've learned a few things since then.
First, this is on a Win-XP-SP3 system, Asus board, 2.93 Pentium-4, 2 G RAM that I built and that has been totally bullet proof ... no exceptions.
Second, I found that the drive had stopped reading only data disks ... it played a video VOB file just fine both in VLC player and Power DVD ... interesting.
Third, I took the "bad" disk to a friend who has three computers in his house ... they all read it just fine.
Lastly, I thought back to anything I had changed or has seen change in the system recently and narrowed it down to changing "Autoplay" to not bug me about what it should do with each disk.
I changed the Autoplay for "Mixed Content" to ask me each time and that did not initially work, but after a re-boot ... it now seems to ... go figure.
So ... I'm not sure at all that I've found the problem, but, for now, the drive is reading data disks again.
I hope I didn't cause anyone to spend any time on this and my apologies for writing before I had exhausted ALL possibilities.