Creating drupal module with CiviCrm

CiviCrm is Constituents Management System which it is integrated in Joomla, wordpress and drupal. We are interested only in drupal 7 and civicrm version 4.2 LTS. Requirements: Civicrm must create a drupal user if the admin creates a contact in civicrm. Note: I managed to syc drupal users to contact in civicrm but not the other way around. I want to find away to create a user in drupal once a contact is created in civicrm. I have good experience in drupal but not in civicrm. It will help if you can provide me a solution using the API or hooks in civicrm. You are required to create either a drupal module or civicrm extension. You are not required to create a website or any other functionality. I have already solved the user registration and memberships. This is the only problem left in the project.