Force Log Off GPO

I’m would like some help on “Force log off group policy.” I am running Windows 2000 Active Directory and have a total of about 30 computers. With this I am also running roaming profiles. My problem is that i haven’t had a successful backup in almost two weeks. I know that in order to back up my AD every body has to be log off the network. I would like to setup the GPO to log every body off at a certain time of day. Please can any body help me?