Time error on windows 2000 server

I am running a windows 2000 server with 6 clients. If we do not keep a program open the server fails to see the users after 16 minutes on the sessions screen. You then have a couple of minutes to open a program or the server times you out and you lose your credentials and have to reboot client to get back on server. I have set the user accounts to no time out limits but it seems the server is timing out the computer not the user. If we leave a program open the server will not time out even though there is no activity. How do I stop the server from timing us out with out leaving a program open and sucking up rescources.