header inc errors

when ever we try to do any thing in our site we get Warning: Cannot modify header information - headers already sent by (output started at /home/squid/public_html/header.inc.php:1)  any ideas what could cause this or how to fix it?

http://squid4duck.org

Make sure aren't any blank lines before the <?php tag and after the ?> tag in the files listed.

When trying to log in my users are seeing this and it will not allow them to log in

I am writing this letter to give my highest possible recommendation for Darren Powless at http://mrpowless.com. I met Darren through a message here at boonex after being taken for a few dollars. Darren first approached me as a reply to my post of a site error I paid to get and could not get the person to fix for me. At our first phone meeting I described the general outline of the errors and Darren went right to work on it. He answered all my questions and appeared intelligent with the in and out of the boonex product.

He walked through each step he was doing and found many errors on my site and explained them very clearly to me. He did this independently - and within 44 minutes he completely removed all errors and we were back up and running. I am very grateful for finding Darren and his expertise, and I was quite impressed at his motivation and independence.

http://mrpowless.com has the best boonex tutorials I have yet seen and his one on one communication and email response is the best yet.