How to find the architecture of OS installed

On Windows Vista and newer you can run the command

wmic os get osarchitecture

to find out whether it's 32 or 64 bit.

On Unix-like OSes you can type uname -m to show the architecture:

$ uname -m
x86_64

Under Windows follow Microsoft's guide:

To find out if your computer is running a 32-bit or 64-bit version of Windows in Windows 7 or Windows Vista, do the following:

    Open System by clicking the Start button, right-clicking Computer, and then clicking Properties.

    Under System, you can view the system type.

If your computer is running Windows XP, do the following:

    Click Start.

    Right-click My Computer, and then click Properties.

        If you don't see "x64 Edition" listed, then you're running the 32-bit version of Windows XP.

        If "x64 Edition" is listed under System, you're running the 64-bit version of Windows XP.

From the console run

set 

and look for the PROCESSOR_ARCHITECTURE value. In my case:

PROCESSOR_ARCHITECTURE=AMD64

Or just print from the beggining

echo %PROCESSOR_ARCHITECTURE%