Overriding user_login

Someone know how to ovveride the user login block in drupal 6 (without form_alter) ?
This code not work in drupal 6:

function phptemplate_user_login($form) {
  $form['name']['#title'] = t('user');
  
  return drupal_render($form);
}

I don't understand why the code not work in D6. Any idea?