site compatability between macs and windows

I'm having some issues getting some sites to render right between macs and pc's. an example is i have some sites that look great in IE, firefox, safari on mac but in IE and firefox on windows their is either some gaps in some css div or the text doesn't look as good on firefox and ie as it does on windows. what is the deal, anyone know or have an idea on how i can fix these dilemas?

part of my issue is the box problem that occurs in IE in just windows only. Just had to set my overflow to hidden, now if i could get the text to display smoother instead of so pixeled.

i'm not saying it is a windows problem, but an IE-windows problem. anyway that is where i found my answer with the broken box theory