Porting 32-bit applications to 64-bit Windows
The PC industry is gradually transitioning from an installed base of primarily 32-bit systems to one of primarily 64-bit systems. In the interim, many application developers will continue to build 32-bit versions of their applications or will provide both 32-bit and 64-bit versions. To ease the effort involved in porting applications and to help encourage adoption of 64-bit computing, Microsoft provides Windows 32-bit On Windows 64-bit (WOW64), an emulation layer that enables 32-bit Windows-based applications to run seamlessly on 64-bit Windows.
Following version offer 64bit as well as 32bit version.
Windows Server 2008
Windows Server 2003
Thanx to WOW64, any existing 32bit application is suppose to run fine on these 64bit operating systems without any changes.