Microsoft Knowledge Base Article 888733 contains the following introduction:

This article lists and briefly describes the x64-based versions of Microsoft Windows Server 2003 and of Microsoft Windows XP Professional x64 Edition.

These versions are the following:

• Microsoft Windows Server 2003, Enterprise x64 Edition

• Microsoft Windows Server 2003, Standard x64 Edition

• Microsoft Windows Server 2003, Datacenter x64 Edition

• Microsoft Windows XP Professional x64 Edition

The x64-based versions of Windows Server 2003 and of Windows XP Professional x64 Edition support Advanced Micro Devices (AMD) AMD64 processors and Intel Extended Memory 64 Technology (EM64T) processors. The underlying architecture of these operating systems is based on 64-bit extensions to the standard x86 instruction set. Therefore, you can run 32-bit and 64-bit programs on x64-processor-based computers that are running an x64-based version of Windows Server 2003 or of Windows XP Professional x64 Edition.