I have been using System 6 on my Macintosh and putting my applications on the Desktop (as opposed to within any folder). After installing System 7, I rebuilt the Desktop and "lost" these applications. However, they appeared again in their correct place, when I opened the hard disk icon.

The issue is that when I try to open a document created by any one of these applications, I get an error message that says, "The application cannot be found."