windows itself should not care what is in the file, it simply uses the extension to pick a related application. Any application should see the file, if the operating system can see it,  but may generate an error trying to load it.

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.