Header problem

Hello Sir,
I am designing login page for user authentication.

Here is my sample code:
index.php

<? ?>
<form action="login.php" method="POST">
Username <input type="text" size="10"
name="username">
Password
<td><input type="password" size="10" name="password">
<input type="submit" name="submit" value="Log In"></td>
</html>

login.php

<?php
// login.php - performs validation
// authenticate using form variables
$status = authenticate($username,$password);//function which returns an
//integer either 0 or 1
// if user/pass combination is correct

if ($status == 1)
{
// initiate a session
session_start();
// register some session variables
session_register("SESSION");
// including the username
session_register("SESSION_UNAME");
$SESSION_UNAME = $username;
// redirect to protected page
header("Location: /securejlt.php");
exit();
}

When I enter username and password in index.php and give submit, am
getting the error "headers already sent".
The manual says that wwe should not have any echo statement before
header statement. I am not using any echo or print statement before header
function is called in my login.php.
I am not able to make out what the problem with my code.

Please help me out in this problem

Regards,
Arun