To determine if you're running an x64 (64 bit) or an x86 (32 bit) machine, please undertake the following steps.

Windows XP:

1. Click Start, and then click Run.
2. Type winmsd.exe, and then click OK.
3. 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 Windows.
   • If the value that corresponds to Processor starts with ia64 or AMD64, the computer is running a 64-bit version of Windows.

Windows 7:

1. Click Start , type system in the Start Search box, and then click System Information in the Programs list.
2. When System Summary is selected in the navigation pane, the operating system is displayed as follows:
   • For a 64-bit version operating system: x64-based PC appears for the System type under Item.
   • For a 32-bit version operating system: x86-based PC appears for the System type under Item.

This information can also be found at: http://support.microsoft.com/kb/827218

Windows 8:

1. Open the Windows 8 Control Panel.  
   Tip: You can check your Windows 8 system type much faster from the Power User Menu but it's probably speedier that way only if you're using a keyboard or mouse. With that menu open, click or touch on System and then skip to Step 4. 
2. Touch or click on System and Security within Control Panel.
   Note: You won't see a System and Security link in Control Panel if your view is set to either Large icons or Small icons. If so, find System and touch or click on it, then skip to Step 4. 
3. With the System and Security window now open, click or touch System. With the System applet now open, titled View basic information about your computer, find the System area, located under the large Windows 8 logo.
4. The System type will say either 64-bit Operating System or 32-bit Operating System.
   Note: The second bit of information, either x64-based processor or x86-based processor, indicates the hardware architecture. It is possible to install a 32-bit edition of Windows 8 or Windows 8 Pro on either an x86 or x64 based system, but a 64-bit edition can only be installed on x64 hardware.Tips:

 Windows 10:

1. Open Settings, and click/tap on the System icon.
2. Click/tap on About on the left side. Under PC on the right side, look to see if your System type is either a 64-bit operating system or a 32-bit operating system.