Quote - windows itself should not care what is in the file, it simply uses the extension to pick a related application

Yes.  I know.

Quote - Any application should see the file, if the operating system can see it

and the application supports the file type

Quote - I would suspect an authority issue , possibly one app is creating files as administrator and the other app doesnt see them if its a user.  I suggest you look at the security on your old files and the two apps involved , and see if that sheds any light on it. That should be in object properties. If windows 7 has it, the admin event viewer might show some security issues.

All the files in question were created by the administrator (me) on the old computer and this one.  Permissions are granted for anyone to create, access, and modify files.  I checked the event viewer but there was nothing regarding a conflict of authorities.