Customizing the user login, register, and password reset pages is fairly simple, and uses the following concepts:

* preprocessing to set variables
* theme register overriding (i think that's the right term)
* creation of one or more theme templates.

<?php
/**
* Registers overrides for various functions.
*
* In this case, overrides three user functions
*/
function yourtheme_theme() {
  return array(
    'user_login' => array(
      'template' => 'user-login',
      'arguments' => array('form' => NULL),
    ),
    'user_register' => array(
      'template' => 'user-register',
      'arguments' => array('form' => NULL),
    ),
    'user_pass' => array(
      'template' => 'user-pass',
      'arguments' => array('form' => NULL),
    ),
  );
}
?>

Notes about that code:

* Change the function name by replacing "yourtheme" with the name of your theme
* The template can be the same for all three. The example above uses a different template for each case: user-login, user-register, and user-pass
* The template names must use a dash, not an underscore
* Note: It's user_pass not user_password

Reff :
http://www.latenightpc.com/blog/archives/2009/02/13/overriding-the-edit-...