Do you know what a Document Type Definition (DTD) is? This is the thing that is specified in the <!DOCTYPE.....> tag at the beginning of your pages. It tells the browser which set of rules you are going to be using and therefore how to display the elements (tags) that make up your pages.
You're probably using the default Document Type Definition (DTD) for ASP.NET which is the XHTML 1.0 Transitional. You may need to change your DTD to an earlier one to get a more consistent look across different browsers. This tutorial is a good place to start
http://aspnetresources.com/articles/doctype.aspx
I had some problems with IE 6 and IE 7 a while back and I found that changing the DTD tag from
[HTML]<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">[/HTML]
To
[HTML]<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">[/HTML]
achieved the results I wanted. If not you'll need to do some more research into the different DTD's.
I hope this helps,
Dr B