Well, as a general rule of web design, you want to design in the *most* standards-compliant browser first, then tweak the design for less-compliant browsers.
This pretty much means you should design for browsers in this order:
1) Firefox (or Mozilla, or Camino, or K-Meleon)
4) Internet Explorer (Windows)
5) Internet Explorer (Mac)
This way, you have a much better chance of having it look right in all browsers. If you start with one of the worst (in terms of compliance) browsers, then it gets harder to tweak it for better (in terms of compliance) browsers.
That being said, I think your problem is one of a bad CSS box model on IE. My best advice for making this work is to tweak your CSS so that it looks right in Firefox, then I would suggest checking out the Box Model Hack
to get IE to play nice with everyone else.