Help me fix my theme

I am not a coder and don’t know any HTML. I built my theme based on the Green Marinee theme by just changing the graphics (I can’t do much else). The site looks great (ehem, well, it displays correctly) in Firefox, but is very messed up in IE. Since, unfortunately, it is still an IE world out there, can someone tell me what might be wrong. With the exception of one cut-and-paste blog entry all the HTML validated.

The site is http://www.ximtc.net.

Thanks,
Jim