Warning: cannot modify header error

I am getting the error; Warning: Cannot modify header information – headers already sent by (output started at /home/jtsme0/public_html/barkington.net/wp-includes/functions.php:1) in /home/jtsme0/public_html/barkington.net/wp-includes/pluggable.php on line 1207

I have deactivated all plugins and it still happens so its not those. I also have checked for any spaces in the first line of the code on functions.php and no space.

Please help.
Regards
Felicia