Rss feedburner

hello i am using wordpress 3.0.4 and i am trying to configure the rss.
My blog is : http://www.cosmoscience.gr

I am trying to configure rss from this site : http://feedburner.google.com/

but i get this error : The URL does not appear to reference a valid XML file. We encountered the following problem: Error on line 209: The entity “raquo” was referenced, but not declared

I validate the feed and i get this : http://feedvalidator.org/check.cgi?url=http://www.cosmoscience.gr/index.php/feed/#l203

can anyone help me please? what are theese errors? how can i fix it to work my rss?