Yes. You can install Project Server 2003 on Windows Server 2000. You will
need to make sure you have all the latest Service Packs, etc.
However, you cannot use Windows SharePoint Services (WSS) with Windows 2000,
so if you want Issues Tracking, Document Control, Threaded Discussions or
Risk Tracking, you will need Windows Server 2003. WSS can be installed on a
different box though.