Åbne login i samme "vindue"

Jeg har en login form som fungerer fint, og det gør registreringsprocessen også.

Alt sammen meget godt men...

I min højre side har jeg login boksen stående, med indtastning af brugernavn og password. Det er ögså fint nok MEN, her er problemet, når brugeren trykker submit åbner den en hel ny side med "du kunne ikke logges ind" eller "login successful" -  i stedet for at åbne det på samme "side" eller hvad man skal kalde det

login_form.php:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body>
<form action="usersystem/checkuser.php" method="post" name="" id="">
  <div align="center">
    <center>
  <table width="155" border="0" cellpadding="4" cellspacing="0" height="28">
    <tr>
      <td valign="top" height="1"><font face="Verdana" size="1">Username</font></td>
    </tr>
    <tr>
      <td width="50" height="25"><input name="username" type="text" id="username"></td>
    </tr>
    <tr>
      <td width="50" height="1" valign="top"><font face="Verdana" size="1">Password</font></td>
    </tr>
    <tr>
      <td width="50" height="25"><input name="password" type="password" id="password"></td>
    </tr>
    <tr>
      <td width="50" height="27"><input type="submit" name="Submit" value="Submit"></a></td>
    </tr>
  </table>
    </center>
  </div>
</form>
</body>
</html>

checkuser.php

<?
/* Check User Script */
session_start();  // Start Session

include 'db.php';
// Conver to simple variables
$username = $_POST['username'];
$password = $_POST['password'];

if((!$username) || (!$password)){
    echo "Please enter ALL of the information! <br />";
    include 'login_form.html';
    exit();
}

// Convert password to md5 hash
$password = md5($password);

// check if the user info validates the db
$sql = mysql_query("SELECT * FROM users WHERE username='$username' AND password='$password' AND activated='1'");
$login_check = mysql_num_rows($sql);

if($login_check > 0){
    while($row = mysql_fetch_array($sql)){
    foreach( $row AS $key => $val ){
        $$key = stripslashes( $val );
    }
        // Register some session variables!
        session_register('first_name');
        $_SESSION['first_name'] = $first_name;
        session_register('last_name');
        $_SESSION['last_name'] = $last_name;
        session_register('email_address');
        $_SESSION['email_address'] = $email_address;
        session_register('special_user');
        $_SESSION['user_level'] = $user_level;
       
        mysql_query("UPDATE users SET last_login=now() WHERE userid='$userid'");
       
        header("Location: login_success.php");
    }
} else {
    echo "You could not be logged in! Either the username and password do not match or you have not validated your membership!<br />
    Please try again!<br />";
    include 'login_form.php';
}
?>

login_succes.php

<?
session_start();

echo "Welcome ". $_SESSION['first_name'] ." ". $_SESSION['last_name'] ."! You have made it to the members area!<br /><br />";

echo "Your user level is ". $_SESSION['user_level']." which enables you access to the following areas: <br />";

if($_SESSION['user_level'] == 0){
    echo "- Forums<br />- Chat Room<br />";
}
if($_SESSION['user_level'] == 1){
    echo "- Forums<br />- Chat Room<br />- Moderator Area<br />";
}

echo "<br /><a href=logout.php>Logout</a>";

?>

Håber på hjælp - tak !