There is a really good article that describes how to do this. You need to go into the registry of the server and make some changes to it. Make sure you back up the registry first though!!!! Then once that it set up you can simply go into Group Policy and set up the client computers to get their time from that server.
http://www.articlesbase.com/operating-systems-articles/how-to-configure-an-authoritative-time-server-in-windows-server-2008-461336.html
