Domain authentication plugin

Does anyone know of a plugin with a function that, passed a username and password as parameters, would authenticate (and do nothing else) that username and password in a Windows 2000 domain? I understand that I can authenticate the user when he/she logs into FileMaker, but I need to authenticate again for specfic functions to ensure the user hasn't left the machine on etc. I am trying to create a sort of "electronic signature" and, rather than build it through FM, if I could just show a custom dialog (which I understand how to do) and pass the collected info to a function that returns a boolean indicating whether the username/password successfully authenticate, I can prevent our system users from having to learn/update multiple usernames/passwords, etc.