I think that users should be verifying their email address first, then they are logged into the site.
If you are not verifying the email address then you can automatically log them in by storing the proper values in session variables. If your logged in rule is based on the userID then you can set the session variable to the just logged in user to have it 'auto login' the user.
On the page that you go to after inserting the user you could also have an authenticate user server behavior that runs before page loads. This will only work if you are not encrypting the password for the user, if you are then this will not work.
Post back with any questions that you have about either of these and I will help with what I can.