Auto-Registration and Login

Hello,

We have already got a validation system that our users will have logged in to before arriving at our internal Drupal installation. That particular database already contains information about the user.

What we want to do is land the user on a pre-Drupal screen that checks the Drupal user table for the existence of their data already. If they are in the Drupal user database then they will be logged in to their account on the Drupal site. If their data cannot be found a script runs to create their row on the Drupal user table and then log them in.

So, my questions are:

What functions will I need to call?
Has anybody tried anything like this before?
Can anybody advise on the auto-login function?

Any help will be gratefully appreciated. I am just about to set off on my research for this, but time is of the essence and I am trying to persuade a massive multi-national manufacturer to use Drupal for an internal system.

Thanks in advance,

Mike Rouse