This is fairly simple.
A quick example:
In your controller you can put
//a test variable
$data["foo"] = 'bar';
$data["page"] = 'a_page'; // this will make sure it loads the views/pages/a_page.php in your template
$this->load->view('templates/login',$data);
And in your views/template/login.php you can put:
<!-- your login template html -->
<html>
...
<!-- include the view you want inside your login template -->
<?php $this->load->view('pages/'.$page);?><!-- As you can see it loads /views/pages/a_page.php -->
<?php echo $foo;?> //This will echo Bar
</html>
or:
<?php $this->load->view('template/login_header');?>
<?php $this->load->view('pages/'.$page);?>
<?php $this->load->view('template/login_footer');?>
views/pages/a_page.php will also know $foo.
This loads another view (views/pages/a_page.php) in your template.
This way you can create all the templates you want, and include your view in those templates.
TIP: Handling headers & footers this way get's unmanageable pretty quickly. And it's still better to use template libraries. Try Phil sturgeon's library