The Windows family supports two different 64-bit architectures.

IA64 versions of Windows 

IA64 is aimed at the Explicitly Parallel Instruction Computing (EPIC) Architecture, and supports the Intel Itanium processor family. Intel's Itanium Processor Family is often referred to as IPF. Windows Server 2003 IA64 and Windows XP IA64 are available right now.

x64 versions of Windows 

x64 is aimed at processors that have 64-bit extensions to the x86 instruction set. This is aimed at processors developed by AMD and Intel (in alphabetical order).

AMD's offering is called AMD64 and it is AMD’s 64-bit platform that extends the x86 instruction set architecture. AMD's FAQ on AMD64 is at http://www.amd.com/us-en/Processors/DevelopWithAMD/0,,30_2252_875_1024,00.html. AMD's 64-bit platform is composed of the Opteron and Athlon 64 processor, which were both released last year.

Intel's offering is called Extended Memory 64 Technology (EM64T) and this is being incorporated into Intel's IA-32 architecture. This addition to the IA-32 architecture started with the release of the new version of the Xeon processor (codenamed "Nocona" for SP and DP, and codename "Potomac" for MP). Intel's FAQ on its EM64T technology can be found at http://www.intel.com/technology/64bitextensions/faq.htm.

Windows Server 2003 x64 and Windows XP x64 are due to be available in H1 of CY '05.