first of all, stop using front page, it'll be the death of you. ;-)
secondly, it's not just mac versus pc, it's actually just internet explorer. i'm using
firefox, and i'm seeing what i would assume to be the same thing that your mac friend is seeing. internet explorer is quirky at times (especially if you don't declare a
DOCTYPE). i highly recommend getting firefox - if only for development purposes.
ok, now on to your problem.
--the menu: your 'button' class uses a % height. you don't want the height to resize with the height of the page, so use a static height, like 18px.
--the main content area: you have #main set to "Z-INDEX: -1;" this pushes the entire main content area behind the body tag. delete the z-index for #main, and it should look fine.