Are you wanting to zoom the whole screen, a section of the screen, a window...? Any one of those will likely involve tracking the user's focus; I'm not sure how you could easily do that with just an Arduino (and it wouldn't be that easy even if you used some other method connected to the PC - like perhaps, a webcam with an IR light source to track the eyeballs and gaze and somehow triangulate that back to a spot on the screen as the "interesting" spot being gazed at to zoom into)...
Good luck - it sounds like a neat project, and if it hasn't been done, I am sure if you are able to get it to work, you could easily sell such a product for those like yourself and others with eyesight problems...
