Help ! "Cannot modify header information "

Bonjour,
J'ai récupéré un script de connection/enregitrement sur un site internet. Apres l'avoir complété, sa marchai, j'arrivai a créé un compte.
Mais, quand je voulai me connecter a la page "membre.php", ben sa me m'etait sa :

Warning: Cannot modify header information - headers already sent by (output started at /public_html/bdd/login.php:1) in /public_html/bdd/login.php on line 57

Warning: Cannot modify header information - headers already sent by (output started at /public_html/bdd/login.php:1) in /public_html/bdd/login.php on line 58

Warning: Cannot modify header information - headers already sent by (output started at /public_html/bdd/login.php:1) in /public_html/bdd/login.php on line 61

Je sais pas du tout d'ou vient le probleme !

VOisi le code de la page "login.php" AU COMPLET !
(J'ai souligné les lignes concerncées)

<?php 
 // Connects to your Database 
 mysql_connect("****", "****", "****") or die(mysql_error()); 
 mysql_select_db("****") or die(mysql_error()); 
 //Checks if there is a login cookie
 if(isset($_COOKIE['ID_my_site']))
 //if there is, it logs you in and directes you to the members page
 { 	$username = $_COOKIE['ID_my_site']; 
 	$pass = $_COOKIE['Key_my_site'];
 	 	$check = mysql_query("SELECT * FROM users WHERE username = '$username'")or die(mysql_error());
 	while($info = mysql_fetch_array( $check )) 	
 		{
 		if ($pass != $info['password']) 
 			{
 			 			}
 		else
 			{
 			header("Location: members.php");
 
 			}
 		}
 }
 //if the login form is submitted 
 if (isset($_POST['submit'])) { // if form has been submitted
 
 // makes sure they filled it in
 	if(!$_POST['username'] | !$_POST['pass']) {
 		die('You did not fill in a required field.');
 	}
 	// checks it against the database
 
 	if (!get_magic_quotes_gpc()) {
 		$_POST['email'] = addslashes($_POST['email']);
 	}
 	$check = mysql_query("SELECT * FROM users WHERE username = '".$_POST['username']."'")or die(mysql_error());
 
 //Gives error if user dosen't exist
 $check2 = mysql_num_rows($check);
 if ($check2 == 0) {
 		die('That user does not exist in our database. <a href=add.php>Click Here to Register</a>');
 				}
 while($info = mysql_fetch_array( $check )) 	
 {
 $_POST['pass'] = stripslashes($_POST['pass']);
 	$info['password'] = stripslashes($info['password']);
 	$_POST['pass'] = md5($_POST['pass']);
 
 //gives error if the password is wrong
 	if ($_POST['pass'] != $info['password']) {
 		die('Incorrect password, please try again.');
 	}
  else 
 { 
 // if login is ok then we add a cookie 
 	 $_POST['username'] = stripslashes($_POST['username']); 
 	 $hour = time() + 3600; 
 setcookie(ID_my_site, $_POST['username'], $hour); 
 setcookie(Key_my_site, $_POST['pass'], $hour);	 
 
 //then redirect them to the members area 
 header("Location: members.php"); 
 } 
 } 
 } 
 else 
 {	 
 
 // if they are not logged in 
 ?> 
 <form action="<?php echo $_SERVER['PHP_SELF']?>" method="post"> 
 <table border="0"> 
 <tr><td colspan=2><h1>Login</h1></td></tr> 
 <tr><td>Username:</td><td> 
 <input type="text" name="username" maxlength="40"> 
 </td></tr> 
 <tr><td>Password:</td><td> 
 <input type="password" name="pass" maxlength="50"> 
 </td></tr> 
 <tr><td colspan="2" align="right"> 
 <input type="submit" name="submit" value="Login"> 
 </td></tr> 
 </table> 
 </form> 
 <?php 
 } 
 
 ?> 

Merci d'avance !