Can’t create tree

Hello,
I’m wanting to create a new tree in an existing forest with Windows 2000 Advance Server. The main forest was created on Windows Server 2003 SBS. I get a “ldap error” whether I use a wizard or DCPROMO. We don’t have internet access yet and I’m assuming there is a service pack to accommedate the new 2003 server in IPsec and replication. Any help or thoughts would be greatly appreciated. Thanks in advance.