[johnnybuck]

I fixed this problem by editing my registry. I was going to use a registry cleaner but decided against it after reading a "HowtoGeek" article advising against them.

- I hit the Windows button and typed in "Run".
- In the "Run" program, I typed "regedit"
- In the Registry Editor, I clicked on "HKEY_CLASSES_ROOT"
- I scrolled down to "Applications" and clicked on it.
- From there, I opened the drop-down menus "ebook-viewer.exe", then "shell", then "open", then "command"
- I then clicked on (Default) under "Name" in the main window.
- I then clicked on the "Edit" menu and then clicked "Modify"
- I edited the path so that it matched the current location of the program (In my case, I changed it from ""C:\Program Files (64)\Calibre2\ebook-viewer.exe" "%1"" to ""C:\Program Files\Calibre2\ebook-viewer.exe" "%1""
- The e-book viewer became available for me to set as a default program.

Hope this helps!