With the help of my son, fixed this problem last night. The solution is to go to windows Default Programs and use the first option "set you default programs", select Windows Photo Viewer, and then Choose defaults. There you can see paint.net set as the default program for .jpg. Just change it back to Windows Photo Viewer and all works fine. Way safer than regedit. Paint.net changes the system default.. a no-no. Could have changed the associations but should have asked. I changed the association of just .jpg and .png back to Paint.net and now Paint.net opens for them but the right-click still uses Windows Photo Viewer as it's supposed to do. Someone should fix that and include instructions for how to correct it.