how rearrange form fields and module....

You can do this by overriding the form layout in you theme. You need to get the form id of your registration form and will need to do some thing like this in the template.php file of your theme:-

function yourtheme_theme() {
   return array(
     // The form ID.
     'user_register' => array(
       // Forms always take the form argument.
       'arguments' => array('form' => NULL),
     ),
	
   );
}

then write following function....

function yourtheme_user_register($form)
{
	$output='';
	$form['User Info']['profile_firstname']['#weight']=10; 
	$output=drupal_render($form);
	return $output;
}

note that you will need to access the field like I did for profile_firstname field here... and set the weight accordingly.

Thanks