I didn't knew someone suffers SAD or RSI. I think it's dificult to suffer SDA in the mediterranian countries. About RSI, I've speak whit sporting friends. Specially the knees of someone of them ache.
Usually I don't sit very well when I work at the computer. I don't remember to break or to change my work at desk position, and frequently my shoulder is sore. I give you five advices if you has just bought a computer. Don't work for a long time. Change your seat position, everytimes. Stretch your arms and your shoulder. Don't bend your wrists. Choose a correct height of your chair.