I would use MySQL to store the user information in a database and then if you make a PHP script which calls the data from the database, you could have a button called "Activate Account" which when clicked would post all the data collected into either the scripts that Zubrag has created, or by using the scripts that I posted in the thread located 2 down from this one I think!!
The setup I have is the whole account is created immediately upon user submition, but my database setup is very unorganised and would be hard to explain. Assuming you have a good knowledge of using PHP and MySQL together, admin activation should be very easy using the scripts I posted.
**Edit**
Here's the link to the topic which includes my scripts....
http://www.zubrag.com/forum/index.php/topic,387.0.html