Exchange 2000 Server Service Pack 3 (SP3) is a cumulative service pack that provides interoperability with Windows .NET Server (Win.NET Server) domain controllers (DCs), fixes to customer-reported problems, and updates based on the Microsoft Trustworthy Computing Initiative. Microsoft recommends that you download and install SP3 on all servers running Exchange 2000.

http://www.microsoft.com/exchange/downloads/2000/sp3/default.asp