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