session_start()

Boa tarde imasters, alguém me ajuda nisso:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs\guia\pagina\login.php:1) inC:\xampp\htdocs\guia\pagina\login.php on line 4

<?php require_once('../Connections/conexao.php'); ?>
<?php
if (!isset($_SESSION)) {
 session_start();
}

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

if (isset($_POST['email'])) {
 $loginUsername=$_POST['email'];
 $password=$_POST['pwd'];
 $MM_fldUserAuthorization = "";
 $MM_redirectLoginSuccess = "sucesso.php";
 $MM_redirectLoginFailed = "error.php";
 $MM_redirecttoReferrer = false;
 mysql_select_db($database_conexao, $conexao);

 $LoginRS__query=sprintf("SELECT login, senha FROM cliente WHERE login=%s AND senha=%s",
 GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text"));

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

	if (PHP_VERSION >= 5.1) {session_regenerate_id(true);} else {session_regenerate_id();}

 $_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 );
 }
}
?>

OBRIGADO!