I have been working recently with Salesforce, and generating Excel files was not working for me.
It should have been as easy as changing the apex:page contentType property, but its output was like:
<table id="j_id0:j_id1" border="0" cellpadding="0"....My problem was using
<apex:column>whatever</apex:column>.Instead, you should use
<apex:column value="whatever" />and it should work.
Hope this helps.