Problems with RIS

Working in a lab environment, I have setup a Windows 2000 Server box with 4 identical workstations. All have PXE compliant NICs. They all can connect to the server for remote installation. I get through the login/naming screens, but when the actual installation of Windows 2000 Pro starts it wants a disk or CD with a previous version of Windows on it in order to upgrade the system. Unfortunately, I’m starting off with a blank drive, and I was under the impression that it was only supposed to do a raw install. No documentation I have seen has mentioned this problem. How do I make it perform a raw install using RIS?