poll/text - cookie problem

I have the following at the top of the voting page:

<?php
// Important! You have to include it before your html code
include_once "/home/httpd/vhosts/xxxxxx.com/httpdocs/pollphp/textfile/poll_cookie.php";
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
(etc, etc)

However after voting I get the following (everything else is fine):

Warning: Cannot modify header information - headers already sent by (output started at /home/httpd/vhosts/xxxxxx.com/httpdocs/poll.php:1) in /home/httpd/vhosts/kimonoboy.com/httpdocs/pollphp/textfile/poll_cookie.php on line 21

What am I doing wrong?

make sure there are no spaces before it, also make sure your cookies are enabled.