yes if u have have one user table or two tables which are 100% duplicate copy of each other at any time...
then u can simply add restrict access server behaviors to all ur pages..

when a user visits such a restricted page he is greeted with a login screen if heis not already logged in..
after login he is thrown back to the source page..
everything should happen transparently without any effort from ur side..
u need to set up
user access to web application use the user registration wizard..
there are help/ tutorials floating around for the same.

Hope this helps
mohnkhan
<http://www.mohitech.com>