|I assume that if one of these files is copied somewhere else it still won't open.|
Are they still showing as jpg files?
It's a long shot but try IrfanView (good free viewer). I have sometimes found that this will open faulty files. If you are lucky and it does so, then resave them with IrfanView. You have no need to permanently associate any files with IrfanView because you can do this from the IrfanView Window using browse.
Always pop back and let us know the outcome - thanks