Dreamweaver Cs y conectar usuarios

[sitoriu]

Hola!, gracias de antemano por la ayuda.

Vereis estoy realizando con wamp una página para validar usuarios mediante el comportamiento de servidor "conectar a usuario". Aunque lo implemento bien cuando ejecuto me sale el siguiente error en la pagina:

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at C:\wamp\www\Blog2\base_login.php:1) in C:\wamp\www\Blog2\base_login.php on line 0

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\wamp\www\Blog2\base_login.php:1) in C:\wamp\www\Blog2\base_login.php on line 0

El codigo de la página es:

<?php virtual('/Blog2/Connections/blog_con.php'); ?>
<?php
if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "")
{
$theValue = get_magic_quotes_gpc() ? stripslashes($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;
}
}

if (!function_exists("GetSQLValueString")) {
function GetSQLValueString($theValue, $theType, $theDefinedValue = "", $theNotDefinedValue = "")
{
$theValue = get_magic_quotes_gpc() ? stripslashes($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)) {
session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
$_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['usuario_frm'])) {
$loginUsername=$_POST['usuario_frm'];
$password=$_POST['pass_frm'];
$MM_fldUserAuthorization = "";
$MM_redirectLoginSuccess = "/Blog2/ok_pass.php";
$MM_redirectLoginFailed = "/Blog2/error_pass.php";
$MM_redirecttoReferrer = false;
mysql_select_db($database_blog_con, $blog_con);

$LoginRS__query=sprintf("SELECT usuario, password FROM usuarios WHERE usuario=%s AND password=%s",
GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text"));

$LoginRS = mysql_query($LoginRS__query, $blog_con) or die(mysql_error());
$loginFoundUser = mysql_num_rows($LoginRS);
if ($loginFoundUser) {
$loginStrGroup = "";

//declare two session variables and assign them
$_SESSION['MM_Username'] = $loginUsername;
$_SESSION['MM_UserGroup'] = $loginStrGroup;

if (isset($_SESSION['PrevUrl']) && false) {
$MM_redirectLoginSuccess = $_SESSION['PrevUrl'];
}
header("Location: " . $MM_redirectLoginSuccess );
}
else {
header("Location: ". $MM_redirectLoginFailed );
}
}
?>

¿Podríais ayudarme?, no tengo mucha experiencia y empiezo a desesperarme.

Gracias