Create app/code/local/Namespace/Module/etc/config.xml  file and add below code

<events>
  <checkout_onepage_controller_success_action> <!-- identifier of the event we want to catch -->
    <observers>
      <checkout_onepage_controller_success_action_handler> <!-- identifier of the event handler -->
        <type>singleton</type> <!-- class method call type; valid are model, object and singleton -->
        <class>createcustomer/observer</class> <!-- observers class alias -->
        <method>Aftersuccess</method>  <!-- observer's method to be called -->
        <args></args> <!-- additional arguments passed to observer -->
      </checkout_onepage_controller_success_action_handler>
    </observers>
  </checkout_onepage_controller_success_action>
</events>

Create app/code/local/Namespace/Module/Model/Observer.php file and add the below code.

<?php
class Namespace_Module_Model_Observer
{

	public function Aftersuccess(Varien_Event_Observer $observer)
	{
		$orderIds = $observer->getData('order_ids');
     	        foreach($orderIds as $_orderId){
			$order = Mage::getModel('sales/order')->load($_orderId);
			$storeId = Mage::app()->getStore()->getId();
			$store = Mage::getModel('core/store')->load($order->getData('store_id'));
			$websiteId = $store->getWebsiteId();
			$billingaddress = $order->getBillingAddress();
			$customer = Mage::getModel('customer/customer')->setWebsiteId($websiteId);
			$customer->loadByEmail($order->getCustomerEmail());

			if(!$customer->getId()) {
				$customer = Mage::getModel("customer/customer");
				$customer   ->setWebsiteId($websiteId)
				            ->setStore($store)
				            ->setFirstname($billingaddress->getFirstname())
				            ->setLastname($billingaddress->getLastname())
				            ->setEmail($billingaddress->getEmail());
				try{
				    $customer->save();
				}
				catch (Exception $e) {
				    Zend_Debug::dump($e->getMessage());
				}

				$address = Mage::getModel("customer/address");
				$address->setCustomerId($customer->getId())
				        ->setFirstname($customer->getFirstname())
				        ->setMiddleName($customer->getMiddlename())
				        ->setLastname($customer->getLastname())
				        ->setCountryId($billingaddress->getCountryId())
						->setRegionId($billingaddress->getRegionId()) //state/province, only needed if the country is USA
				        ->setPostcode($billingaddress->getPostcode())
				        ->setCity($billingaddress->getCity())
				        ->setTelephone($billingaddress->getTelephone())
				        ->setFax($billingaddress->getFax())
				        ->setCompany($billingaddress->getCompany())
				        ->setStreet($billingaddress->getStreet())
				        ->setIsDefaultBilling('1')
				        ->setIsDefaultShipping('1')
				        ->setSaveInAddressBook('1');
				 
				try{
				    $address->save();
				}
				catch (Exception $e) {
				    Zend_Debug::dump($e->getMessage());
				}
			}
		}
	}	
}