I read with dismay that TomDart has a backup CD that is,

The opened image is the main one.  Damaged.  Totally not readable.

But why does the thumbnail (if I choose thumbnail in the folder view
instead of list, etc.) show what appears a full shot?

Let me try to answer that question first ... on the CD, do you see a file called
"thumbs.db"?  -- that is a small database of the image previews.  If you had
chosen to view thumbnails before the backup was made, that file was probably
created from a good copy of the image (JPEG).  BTW, in order to see the
thumbs.db file, you might have to change some folder settings ...
In Windows Explorer, click ... Tools -> Floder Options -> View -> Advanced Settings
-> Hidden files and folders CHECK Show hidden files and folders

Another question ... on that older PC, did you have some improper shutdowns? 
The way you know about those is that on the next boot, you see that Windows is
checking the disk (HDD) for errors.  We have all had that happen ... as a rule, did
you allow that process to complete, and did you allow it to correct each error? 

--
Martin