Core not autoloading

[eluser]CrazyMerlin[/eluser]
Hey people!

I have a file called loader.php which acts as an AJAX layer to the system. The file contains basic functions, no class.
In the registration form for a site I am building I check for the username existing when they enter one in the form.
I have a user controller which requests the user model to look for the username. However, the user controller throws an error when I instantiate it. If I explicitly require it before I instantiate it all is well, until it extends Controller which then throws the error. If I explicitly require Controller I would then need to do the same for all base classes used in Controller.

Is there a file with an autoload function that I need to require in my bootstrap, or will I need to create an __autoload() magic function?

Thanks,
Paul.