Hello Readers,
This tutorial will guide you about controller overriding using config.xml. Here i am giving an example "to override Customer AccountController".

Lets start by creating the following folders:
app/code/local/Custom/Registration/etc
app/code/local/Custom/Registration/controllers

1. Create a file config.xml (under /app/code/local/Custom/Registration/etc).

<?xml version="1.0"?>
<config>
    <modules>
        <Custom_Registration>
            <version>1.0.1</version>
        </Custom_Registration>
    </modules>

        <frontend>
            <routers>
                <customer>
                    <args>
                        <modules>
                            <custom_registration before="Mage_Customer">Custom_Registration_Customer</custom_registration>
                        </modules>
                    </args>
                </customer>
            </routers>
        </frontend>
</config>

<custom_registration before="Mage_Checkout">Custom_Registration_Customer</custom_registration> tag tell Magento that firstly load our custom controller files before the mage_customer core files.

2. Create a controller file "app/code/local/Custom/Registration/controllers/Customer/AccountController.php" and paste the following contents in that file.

<?php
 require_once 'Mage/Customer/controllers/AccountController.php';
class Custom_Registration_Customer_AccountController extends Mage_Customer_AccountController
{
        public function createAction()
    {
        echo "Account controller is overriden"; die;
       
    }

}

3. Next step is to tell Magento to load our module and to do this we need to create a file Custom_Registration.xml under /app/etc/modules and paste the following code:  

 <?xml version="1.0"?>
    <config>
        <modules>
            <Custom_Registration>
                <active>true</active>    //  is active or not
                <codePool>local</codePool> //location of the module i.e inside the local folder
            </Custom_Registration>
        </modules>
    </config>