problema form

[Bruce5000]

Ciao, ho questa form:

<form name="form1" method="post" action="main.php">
  

  

  

  <table width="40%"  border="1" align="center" cellpadding="0" 
cellspacing="0" bordercolor="#000000" bgcolor="#FFFFFF" class="bannerbox">
    <tr>
      <td><table width="100%"  border="0" align="center">
        <tr>
          <td colspan="3" bgcolor="#000000"><span 
class="style1"></span><span class="style1">Admin Login 
</span></td>
          </tr>
        <tr>
          <td colspan="3"></td>
        </tr>
        <tr>
          <td></td>
          <td colspan="2">[b]Inout Mailing List  Manager ADMIN Login! 
[/b]</td>
          </tr>
        <tr>
          <td width="5%"></td>
          <td width="15%"></td>
          <td width="80%"></td>
        </tr>
        <tr>
          <td></td>
          <td>Username</td>
          <td><input name="username" type="text" id="username">
          </td>
        </tr>
        <tr>
          <td></td>
          <td>Password</td>
          <td><input name="password" type="password" id="password">
            </td>
        </tr>
        <tr>
          <td></td>
          <td></td>
          <td><input type="submit" name="Submit" value="Login!"></td>
        </tr>
        <tr>
          <td></td>
          <td></td>
          <td>Please enter your username and password

            above
            and click Login! 

            </td>
        </tr>
      </table></td>
    </tr>
  </table>
</form> 

Questa è lo script php che gestisce il form:

<?php
include("config.inc.php");
if(isset($_POST['Submit'])){
 $username == $_POST['username'];
 $password == $_POST['password'];

if(($username == "jessy") && ($password == "peie")){
header("Location: main.php");
}
else{
   echo "<script type=\"text/javascript\">";
   echo "alert(\"Username e password errati!\")";
   echo "</script>";
}
}
?>

Nel form, quando si clicca sul pulsante, si apre la pagina main.php a
prescindere da quali username
e password vengono inseriti o anche se le textbox vengono lasciate vuote.
Potete darmi una mano?