Vedr: Fejlmelding ved start af Session

Hvorfor får jeg følgende fejl hver gang jeg prøver at starte en session

Warning: session_start()[<a href='function.session-start'>function.session-start]</a>:Cannot send session cookie - headers already sent in C:\Inetpub\wwwroot\php\login.php

<?php
session_start();      //fejl her

  if (isset($_POST['username']) && isset($_POST['password'])) {

    $username = $_POST['username'];
    $password = $_POST['password'];

    $connection = mysql_connect('localhost', 'root', '')

    if(!connection)
    {
          die('Kunne ikke forbinde til                databasen: '.mysql_error());
    }

    mysql_select_db('Forening', $connection);

    $sql = "SELECT * FROM person WHERE Username='$username' "
            ." AND Password='".sha1($password)."'";
            //." AND Password='$password'";

    $result = mysql_query($sql, $connection);

//valid username sættes
    if (mysql_affected_rows() > 0){
       
            $_SESSION['valid_user'] = $username;
       
            $redirect = 'location: HentPersonFraForeningstabel.php?RedirectFrom='.basename(__FILE__);
            header($redirect);
        }
    mysql_close();

    $_SESSION['sql'] = $sql;

  }
      //tjek for at se krypteringen af 1234 og sql-streng 
    //print sha1('1234');
    //print $sql;
?>