Completly off-topic, but I learned something, I never knew you could rotate an image from Explorer... then again, I only have XP at work, and I almost never use thumbnail view....
Back on topic, I'd suggest GIMP too, it's over-powered for just rotating an image, but it'll do it and much more, and it's probobly already installed.
As for how to rotate an image in GIMP:
- Click "Image" from the menu bar
- Click "Transform"
- Click whichever option you want