I've just done a bit of tinkering with my Windows Photo Gallery, and the best that I can come up with is this:
1. Go to Photo Gallery and click on your picture so that it appears, alone, on your screen.
2. Click on Fix tab (at top of screen). List of options appears on right of screen.
3. Click on Crop Picture. Crop frame appears on your photo.
4. Click on Rotate Frame (just below Crop Picture).
The crop frame will rotate to landscape, but unfortunately it means that you will lose some of the picture at the top and bottom. Sorry, but without a more-advanced photo editing program that's the best that I can think of.