Easy way to check existence of a user in DB ? ( Newbie )

[eluser]Unknown[/eluser]
Hi, i'm a CodeIgniter newbie, i'm developing a simple authentication system.

My validation controller (target of login form) looks like this:

Code:

$username = $this->input->post('username');
$password = $this->input->post('password');

// Here starts the code for checking the existence of the username && password...

Is there a simple way to check the existence of that user in that table ? I did not found it on the user guide.

Thanks in advance!

(Sorry for my english)