As announced at WinHEC, release versions of Windows x64 Server and Client editions are here. If you already have an x64 computer running Windows XP Professional (32-bit), find out how you can exchange your 32-bit edition of Windows XP Professional for Windows XP x64 Edition. A similar program is available to customers who purchased the 32-bit version of Windows Server 2003 with x64 hardware and would now like to exchange their software for the x64 version of Windows instead. Find out more about the Technology Advancement Program.

http://www.microsoft.com/windowsserver2003/64bit/x64/overview.mspx