Automatically log users off

Does anyone know of an application, or service which can be installed on an NT 4.0/W2K network that will log users off after a certain time? I would like to be able to log all users off at 12 AM every night so that my backup works correctly. I currently have 1 PDC and 1 BDC, but they are only used for authentication, not applications. I know you can do this with Novell, just not sure how to do it with NT. Any help, GREATLY appreciated! Thanks, Roger