Could any kind soul help me please?

[eluser]zen333[/eluser]
I am new to CI and have very little knowledge on website programming. I followed the video tutorial on "Hello World" but got an error msg :Fatal error: Class 'Controller' not found in C:\wamp\www\CI\system\application\controllers\blog.php on line 3

My codes:
<?php

class Blog extends Controller {
}
function index()
{
echo 'Hello World'; }


?>

$config['base_url'] = "http://localhost/CI/"; and I have copied all my CI files to c:www/wamp.... Please help me! Thanks a million!