OK, I did the CCleaner Registry cleanup (it found a whole lot of stuff!) Then I tried to open with the viewer. and it still said that there needed to be the proper extension.
I then wanted to check what the actual DOS filename was. I do this by moving the file to a C: level directory and then use the Command Prompt (i.e., DOS Window) app, and do a DIR command there. I noticed that one of the files had a double 'epub' extension, and so to change the extension, I need the base filename to have no spaces, so I changed that filename as such.
To my surprise, the double click Open worked!
: But then the other file, that had the regular single 'epub' extension did not.
On a hunch, I came to the conclusion that the only thing different between the files is that one had spaces in the name, while the other did not, so I went and changed the names back to the original ones - although now with underscores in lieu of spaces - and they worked.
:
So there it is - there is a bug in the program in that it does not allow spaces in the filename (although using the Calibre library to spawn the viewer app, such names with spaces does work.)