This page contains the following errors: error on line 1 at column 40: XML decl

Hi!;

One of my site was hacked some time ago, and I recovered almost everything, but now when I try to generate a sitemap with your plugin I get this result when checking the file:

This page contains the following errors:

error on line 1 at column 40: XML declaration allowed only at the start of the document
Below is a rendering of the page up to the first error.

The real content is like this:

<iframe src="http://*.co.tv/?go=1" width="1" height="1"></iframe><?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="http://*.com/wp-content/plugins/wordpress-seo/css/xml-sitemap.xsl"?>
-----------------------------------------------------------------------^

Yes, is obvius, this Iframe code is messing it all, somebody knows where I must search to rid off of this error?

Thanks! and great plugin!