rjackowens :
So do programs in "Program Files (x86)" still take advantage of 64 bit Windows?
All programs that were compiled as x64 will run as x64 processes and take advantage of your 64 bit CPU. These programs can be placed anywhere in your filesystem as long as they have permission to be executed. Programs compiled as x64 will not run on a 32 bit operating system.
All 32 bit x86 Windows programs should run on both 32 bit and x64 versions of Windows. 16 bit programs will only run on 32 bit x86 Windows. 32 bit x86 Windows programs running on x64 Windows should run slightly slower than on 32 bit x86 Windows. This is because they are run using Microsoft's WoW64 (Windows on Windows 64-bit,) a light weight compatibility layer. These programs can be placed anywhere in your filesystem as long as they have permission to be executed.
To clarify, x64 is AMD's 64 bit architecture and is also often called AMD64. Intel developed their own 64 bit architecture called Itanium (often called IA-64), but licensed AMD's 64 bit architecture after Itanium failed to catch on in the market place. Intel calls their implementation of AMD64 EM64T. AMD64 and EM64T are compatible. I try to be careful about this terminology because there were three versions of Windows XP. Windows XP, Windows XP 64 bit Edition, and Windows XP x64 Edition. Windows XP is the standard 32 bit x86 version, Windows XP 64 bit Edition is the version for Itanium CPUs, and Windows XP x64 Edition is the version for x64 cpus. In addition, there are other 64 bit and 32 bit architectures besides 64x and x86.
Learn more:
https://en.wikipedia.org/wiki/X86-64
http://en.wikipedia.org/wiki/WoW64
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