[Resolvido] problema com session_start

alguém pode me dizer qual a razão desse erro ? já procurei em todo canto e não acho

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at C:\xampp\htdocs\projetoFinal\senha.php:10) in C:\xampp\htdocs\projetoFinal\senha.php on line 23

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp\htdocs\projetoFinal\senha.php:10) in C:\xampp\htdocs\projetoFinal\senha.php on line 23

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\projetoFinal\senha.php:10) in C:\xampp\htdocs\projetoFinal\senha.php on line 31

<?php
ob_start();

$login = davi;
$senha = 123456;

if ($login == $_POST['usuario'] && $senha == $_POST['senha'])

{

$validacao = "1";
$usuario = $_POST['usuario'];

session_start();
$_SESSION[usuario] = $usuario;
$_SESSION[validacao] = $validacao;

header ("Location: admin.php");
}

else {

?>
<script type="text/javascript">

alert("Login ou senha incorreta")

</script>
<?php
echo "<a href=index.php>VOLTAR</a>";
}
?>