Barry, first I want to be sure that there isn't a setting I missed when I set it up for you. In Mail, click on the Edit menu, then go down to Attachments. The middle item in the submenu is Always Send Windows-Friendly Attachments, and it should have a check mark to the left. If it doesn't, click on it, and that will take care of it.

If it already has the check mark, or if it doesn't but these people still can't open your attachments, the only way we're going to figure it out from this end is to determine what these folks have in common with each other that's different from your other PC recipients. It could be their email service and/or the program (and version of that program) they're using to read it.