class Namespace_Module_Model_Observer {

  public function customerRegisterSuccess(Varien_Event_Observer $observer) {

      $event = $observer->getEvent();

      $customer = $event->getCustomer();

      $email = $customer->getEmail();

      if($email) {

          //code to handle if customer is successfully registered

      }

  }

}

<?php $order = Mage::getModel('sales/order')->loadByIncrementId($this->getOrderId());

$quoteId = $order->getQuoteId();

$quote = Mage::getModel('sales/quote')->load($quoteId);

$method = $quote->getCheckoutMethod(true);

$customer_email = $order->getCustomerEmail();

if ($method == 'register'){ ?>

//code to handle if customer just registered to your site

<?php } elseif($method == 'guest') {?>

//code to handle if customer is guest

<?php } else { ?>

//code to handle for logged in customer

<?php } ?>

$_customerId = Mage::getSingleton('customer/session')->getCustomerId();

$lastOrderId = Mage::getSingleton('checkout/session')->getLastOrderId();

$order = Mage::getSingleton('sales/order');

$order->load($lastOrderId);

$_totalData =$order->getData();

$_sub = $_totalData['subtotal'];

$_orderEmail = $_totalData['customer_email'];

$_orderNumber = $_totalData['increment_id'];

$e = $email; //provided by guest user

$store = Mage::app()->getStore();

$websiteId = Mage::app()->getWebsite()->getId();

$customerObj = Mage::getModel('customer/customer');

$customerObj->website_id = $websiteId;

$customerObj->setStore($store);

$prefix = "mag";

$pwd = uniqid($prefix);

$session = Mage::getSingleton('checkout/session');

$fname = $session->getFirstname();

$lname = $session->getLastname();

$customerObj->setEmail($e);

$customerObj->setFirstname('Guest');

$customerObj->setLastname('Guest');

$customerObj->setPassword($pwd);

$customerObj->save();

$customerObj->sendNewAccountEmail('confirmed'); //auto confirmed