ismap / imagemap help

I am using the ismap tag and for the most part, it works fabulously. However, recently I went on a co-workers computer and noticed that the ismap coordinates are incorrect. the image is set to be 800 x 400 but on his computer it thinks the image is 100 x 500!!! So far it's the only computer that does that. Has anyone else encountered this problem and how to solve it?

thanks a mill,
sharyn