System is running in 32-bit or 64-bit?

Microsoft Windows
Click Start, and then click Run.
Type winmsd.exe, and then click OK.
When System Summary is selected in the navigation pane, locate Processor under Item in the details pane. Note the value.
If the value that corresponds to Processor starts with x86, the computer is running a 32-bit version of the Windows operating system.
If the value that corresponds to Processor starts with ia64 or AMD64, the computer is running a 64-bit version of the Windows operating system
--------------------------OR-------------------------------------------
Click Start, and then click Run.
Type sysdm.cpl, and then click OK.
Click the General tab. The operating system appears as follows:
For a 64-bit version operating system: Microsoft Windows XP Professional x64 Edition Version <> appears under System.
For a 32-bit version operating system: Microsoft Windows XP Professional Version appears under System.


IBM-AIX
getconf -a grep KERN

HP-UX
getconf KERNEL_BITS

Linux
uname -a