Почему при написании

<?php

           session_start();

           ?>

Выдается ошибка

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at C:Documents and Settingssaitform_reg_mak.php:9) in C:Documents and Settingssaitform_reg_mak.php on line 172

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:Documents and Settingssaitform_reg_mak.php:9) in C:Documents and Settingssaitform_reg_mak.php on line 172

В php.ini

register_globals = On