Update on 80 Days. I haven't heard back from Frogwares, but I now have the game running perfectly on my Windows 7 computer. How I got it working is a bit convoluted. It involved installing the German version of 80 Days which a friend of mine had, which must be a different build than the one that has been giving people here trouble who have the English version. This version installed and played with no issues aside from the fact that the spoken voice and subtitles were both in German, with no options for English.
I also discovered that in fact I did still have my old CD of 80 Days, buried in the back of a closet full of old games and CDs. I was able to copy the voice and text files from this CD and replace the German voice and text files with the English ones. Now the game is running in English, with English subtitles, and no technical problems whatsoever.
Anyway, I wanted to let you know what I did to get it working.
This isn't the first time that a European version of a game turned out to be a different build than the version I had in the States, and for whatever reason proved to be easier to get running.