You're floating.
It's a common problem with ie anything. ie7 was supposed to have worked it all out, but as you can see they haven't worked everything out.
Before I tell you how to fix this, I wanted to mention that your page title still says zen cart. you need to change that so that google and other search engines will start indexing you properly.
I'll look at your code to give you the work around, but try adding a height to your header in % instead of px.



