Creation of new user overwrites existing user accounts

Dear people,

I have a problem with my Drupal installation. At this moment our user database contains more than 900 users. When i want to create a new user, the new userid is not 901 (as expected) but 12. So the creation of a new user overwrites an old existing user.

I looked into the Drupal database and noticed that the user id (in the users table) is not AUTO_INCREMENT. Is this correct? (it seems an error to me..with AUTO_INCREMENT it should be impossible to overwrite old records by inserting new ones)

How to reproduce?
-Create a new user
-Drupal will send a email with an one-time activation link
-The new user clicks the link
-The new user can see the record of another user with his/her own email address

My Drupal version = Drupal 5.7
My Database = mysql 5.0