At Its Core

• According to Microsoft Support, the x64 edition of Windows XP is based on the Windows Server 2003 code tree. Windows XP Professional x64 was intended to offer the same feature set and user interface as the standard Windows XP Professional edition with service pack 2. Windows XP Professional x64 is not compatible with the service packs released for 32-bit editions of Windows XP. Instead, it receives unified service packs with Windows Server 2003, as seen in the technical overview for Service Pack 2 for Windows Server 2003.
  
  

More Memory

• Support for memory spaces larger than 4 gigabytes (GB) is the most obvious difference many users will notice when upgrading to a 64-bit operating system. According to Microsoft, Windows XP Professional x64 supports up to 128 GB of physical RAM and 16 terabytes of virtual address space. Individual processes are allowed as much as 8 terabytes of virtual address space, compared with only 3 GB in 32-bit Windows.
  
  

Improved Performance

• Sixty-four bit processors are better suited for certain operations such as those found in encryption or audio/video-encoding applications. Applications that are optimized to use these advantages can operate significantly faster, but a 64-bit operating system is required to run them. Microsoft also states that Windows XP Professional x64 uses additional registers on 64-bit processors that 32-bit Windows versions cannot. This allows more information to be stored on the processor itself, reducing the frequency of time-consuming calls to RAM.
  
  

Greater Security

• Viruses and rootkits that are aimed specifically at 32-bit system files cannot affect 64-bit versions of Windows. According to Microsoft, Windows XP Professional x64 also includes Patch Guard technology, which prevents third-party programs from patching the Windows kernel. This closes a common avenue used by many rootkits to infect Windows systems. Patch Guard technology is not available in 32-bit versions of Windows XP.
  
  

Compatibility

• According to Microsoft, Windows XP Professional x64 uses the WOW64 subsystem to provide full compatibility with 32-bit applications. These applications perform just as well, if not better, in WOW64 when compared against 32-bit versions of Windows XP.
  
  Sixteen-bit applications are no longer supported in Windows XP Professional x64. This means that programs written for MS-DOS or Windows 3.1 will not run. Microsoft points out, though, that some popular 16-bit installers are supported as exceptions to this limitation.
  
  

Drivers

• Windows XP Professional x64 is not compatible with 32-bit hardware drivers, requiring 64-bit versions instead. Sixty-four bit drivers have grown increasingly more common as new 64-bit Windows operating systems have become popular, but Windows XP Professional x64 is an outdated operating system, and 64-bit drivers were rare when it was initially released.
  
  Many manufacturers do not consider it cost effective to write drivers for the 64-bit version of Windows XP, making it difficult to find working drivers for many pieces of hardware. For this reason, Windows XP Professional x64 can sometimes be user unfriendly. Printers, cameras, sound cards and many other devices may simply not work for this operating system.