With its new 64-bit versions, Windows XP Professional x64 Edition and Windows Server 2003 x64 Editions, Microsoft is providing users with the option of running both 32-bit and 64-bit applications on the same system. The operating system is designed to work on newer 64-bit processors from Advanced Micro Devices and Intel Corp.Read more at PC World
Microsoft releases Windows XP Professional x64 RC2

Microsoft released the second release candidate of Windows XP Professional x64 Edition, as well as Service Pack 1 for Windows Server 2003 and Windows Server 2003 x64 Editions.