How do I remove elements in the source code that make my feed invalid?

Hi,

My blog has been working fine, but as of my last post Feedburner tells me it is invalid because of some elements that I can see in the first few lines when I view the source code. The common explanation for such unwanted code, apparently, is cutting and pasting text from other composing platforms like Word, but I haven’t done that.

Where does this unwanted code come from? Where exactly is it located in my blog? (I don’t see it in any of the theme files.) And how do I get rid of it?

One possibly related note: I’m using the Dawn to Dusk theme, which displays fine in Firefox and Safari, but not in IE. When I experimented by switching to Twenty Ten, it looked a little better in IE but was still off-center. The problematic code still showed when I tried viewing it.

My blog is at http://www.acrossthepage.net. Any help would be appreciated!