What? Is that right??? So, if the image thumb in Windows Explorer is shown in wrong orientation, what would be the lossless way of viewing the image in the correct orientation?
Sorry, I should have been a little clearer. If you use, for example, the Windows Photo Viewer as your Preview application in Windows, and rotate the image using Windows Photo Viewer, the file will be re-encoded, and will lose quality. You can easily test this. Just note the "Date modified" on a file, open it in Windows Photo Viewer (Preview it) and rotate it. The "Date modified" changes to the current date and time. The same thing happens if you right click on a thumbnail in Windows Explorer and select "Rotate Clockwise" or "Rotate Counterclockwise".
The lossless way to view a file is to use a codec that will honours the orientation tag in the file. FastPictureViewer and its codecs does that.
And there seems to be a system to this. Portraits taken with my Nikon D200 are more often viewed with wrong orientation, while my wife's old Canon Ixus is almost always shown correctly.
I wondered about that, but you implied that you couldn't see a pattern to the problem. It is entirely possible that the Nikon D200 isn't writing the correct orientation tag, and the IXUS is. You should check that for the Nikon photos in MC. Also check the tags
in the file (rather than just in the library) with MC, to see if any other information is in there. However, many of the photos I checked on my client are from a Nikon D7000, and they rotate correctly, so I would expect the Nikon photos to work. But then, I have FastPictureViewer and its codecs installed.
By the way, you should probably try with the "Microsoft Camera Codec Pack" I linked to first. You may not need FastPictureViewer, although as you say, new software is always fun.