Microsoft Knowledge Base Article 875423 contains the following introduction:

This article describes new functionality that is introduced in Microsoft Windows Server 2003 Service Pack 1 (SP1) and in x64-based versions of Windows Server 2003. Windows Server 2003 with SP1 and x64-based versions of Windows Server 2003 let you run 32-bit programs on a 64-bit Windows Server 2003-based server cluster.