Firefox/IE Differences in Appearance

I managed to implement a new theme and customize it the way I wanted it. See it here: http://drbrooker.net It looks great in Firefox, which I use, but when I went to see it in IE it was messed up. ie. right sidebar to the right when not viewed in full screen. Is there a way to determine in your code what needs to be changed so that IE will read it properly? How can I know what to tweak so it looks the same in both. Thanks.

Darrin