WoW64

WoW64 (Windows 32-bit on Windows 64-bit) is a subsystem of the Windows operating system capable of running 32-bit applications and is included on all 64-bit versions of Windows - including Windows XP Professional x64 Edition, IA-64 and x64 versions of Windows Server 2003, as well as 64-bit versions of Windows Vista, Windows Server 2008, Windows 7 and Windows 8. In Windows Server 2008 R2 Server Core, it is an optional component.
WoW64 is designed to take care of many of the differences between 32-bit Windows and 64-bit Windows, particularly involving structural changes to Windows itself.

Translation Libraries

The WoW64 subsystem comprises a lightweight compatibility layer that has similar interfaces on all 64-bit versions of Windows. It aims to create a 32-bit environment that provides the interfaces required to run unmodified 32-bit Windows applications on a 64-bit system. Technically, WoW64 is implemented using three dynamic-link libraries (DLLs):

1. Wow64.dll, the core interface to the Windows NT kernel that translates between 32-bit and 64-bit calls, including pointer and call stack manipulations
2. Wow64win.dll, which provides the appropriate entry-points for 32-bit applications
3. Wow64cpu.dll, which takes care of switching the processor from 32-bit to 64-bit mode

Registry and File System

The WoW64 subsystem also handles other key aspects of running 32-bit applications. It is involved in managing the interaction of 32-bit applications with the Windows components such as the Registry, which has distinct keys for 64-bit and 32-bit applications. For example HKEY_LOCAL_MACHINE\Software\Wow6432Node is the 32-bit equivalent of HKEY_LOCAL_MACHINE\Software (although 32-bit applications are not aware of this redirection). Some Registry keys are mapped from 64-bit to their 32-bit equivalents, while others have their contents mirrored, depending on the edition of Windows.

The operating system uses the %SystemRoot%\system32 directory for its 64-bit library and executable files. This is done for backward compatibility reasons, as many legacy applications are hardcoded to use that path. When executing 32-bit applications, WoW64 transparently redirects 32-bit DLLs to %SystemRoot%\SysWoW64, which contains 32-bit libraries and executables. 32-bit applications are generally not aware that they are running on a 64-bit operating system. 32-bit applications can access %SystemRoot%\System32 through the pseudo directory %SystemRoot%\sysnative.

There are two Program Files directories, both visible to both 32-bit and 64-bit applications. The directory that stores the 32 bit files is called Program Files (x86) to differentiate between the two, while the 64 bit maintains the traditional Program Files name without any additional qualifier.

Incompatible Applications

32-bit applications that include only 32-bit kernel-mode device drivers, or that plug into the process space of components that are implemented purely as 64-bit processes (e.g. Windows Explorer) cannot be executed on a 64-bit platform. Service applications are supported.
The SysWOW64 folder located in the Windows folder on the OS drive contains several applications to support 32-bit applications (e.g. cmd.exe, useful to register 32bit windows services, odbcad32.exe, to register ODBC connections for 32-bit applications). 16-bit legacy applications for MS-DOS and early versions of Windows are usually incompatible with 64-bit versions of Windows Vista, 7 and 8, but can be run on a 16-bit or 32-bit Windows OS via Microsoft Virtual PC or DOSBox. 32-bit versions of Windows XP, Vista, 7, and 8, on the other hand, can usually run 16-bit apps with few to no problems.
The component that makes this possible, Windows on Windows 32-bit, is replaced in 64-bit Windows OSs by WoW64, rendering nearly all 16-bit apps unexecutable.

Internet Explorer is implemented as both a 32-bit and a 64-bit application because of the large number of 32-bit ActiveX components on the Internet that would not be able to plug into the 64-bit version. The 32-bit version is used by default and the 64-bit version cannot be set to be the default browser.