The 64-bit operating system has been long awaited by both Intel and especially Advanced Micro Devices, whose 64-bit Athlon and Opteron microprocessors have been forced to run on beta versions of the operating system. Linux distribution vendors, on the other hand, have offered 64-bit versions for months.More info at PC Mag
Microsoft releases 64-bit operating systems to manufacturers

Microsoft today shipped 64-bit versions of Windows Server 2003 and Windows XP to manufacturers.