Ah, good old SMS (Shaky Monitor Syndrome). I used to fix this all the time for people when I worked in IT.
Yes, there is magnetic shielding you can get, but it's damned expensive. Search for "mu-metal". However, by *far* the easiest solution is to keep the fan a little bit further away, and make sure it's on a different electrical circuit if at all possible. You might also try a smaller fan, or just a different model; some are much worse than others in terms of electromagnetic interference generation.
Oh, and you shouldn't have any long-term ill effects, unless it's intense enough to be seriously distorting the screen (which may end up having effects similar to a permanent magnet).