[Resolvido] Erro em Login

Boas pessoal estou a desenvolver um sistema de login com a ajuda DW, mas dá os seguintes erros:

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at C:\AppServ\www\exemplos\usuarios\login.php:1) in C:\AppServ\www\exemplos\usuarios\login.php on line 35

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\AppServ\www\exemplos\usuarios\login.php:1) in C:\AppServ\www\exemplos\usuarios\login.php on line 35

Na linha 35 está:

session_start();
Por cima desta linha tenho:

<?php require_once('Connections/conexcao_user.php'); ?>
<?php
if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "")

{

 if (PHP_VERSION < 6) {
 $theValue = get_magic_quotes_gpc() ? stripslashes($theValue) : $theValue;
 }
 $theValue = function_exists("mysql_real_escape_string") ? mysql_real_escape_string($theValue) : mysql_escape_string($theValue);
 switch ($theType) {
 case "text":
 $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
 break;
 case "long":
 case "int":
 $theValue = ($theValue != "") ? intval($theValue) : "NULL";
 break;
 case "double":
 $theValue = ($theValue != "") ? doubleval($theValue) : "NULL";
 break;
 case "date":
 $theValue = ($theValue != "") ? "'" . $theValue . "'" : "NULL";
 break;
 case "defined":
 $theValue = ($theValue != "") ? $theDefinedValue : $theNotDefinedValue;
 break;
 }
 return $theValue;
}
}
?>
<?php

// *** Validate request to login to this site. if (!isset($_SESSION)) {

Será que alguém me pode ajudar a resolver estes erros??? Fico a aguardar a vossa ajuda...