Various Internet sites have information about Active Directory Service Interfaces (ADSI). Microsoft's main ADSI site and the 15 Seconds Web site also contains useful ADSI links and an ADSI mailing list.
You can download the ADSI 2.5 beta kit from http://www.microsoft.com/ntserver/ nts/downloads/previews/ADSI25/default.asp. You'll need Windows NT 4.0 Service Pack 3 (Build 1381: SP3), Windows 98, or Win95 to install the kit. NT 5.0 Beta 2 or later includes ADSI 2.5.