32-bit Windows or 64-bit Windows?

Source
The Microsoft® Windows® Malicious Software Removal Tool is different for 32-bit Windows and 64-bit Windows. Here is how to tell which you are on.

Vista

Option A: Control Panel

1. Click Start, type system in the Start Search box, and then click system in the Programs list.
2. Either 64-bit Operating System or 32-bit Operating System appears for the System type under System.

Option B: System Information Window

1. Click Start type system in the Start Search box, and then click System Information in the Programs list.
2. Select System Summary in the navigation pane; either x64-based PC or x86-based PC appears for the System type under Item.  x86 is the 32-bit system.
   

Windows XP

Option A: View System Properties in Control Panel

1. Click Start, and then click Run.
2. Type sysdm.cpl, and then click OK.
3. Click the General tab. Under System, if you see Microsoft Windows XP Professional x64 Edition Version you have a 64-bit system.  If you see only Microsoft Windows XP Professional Version it is the 32-bit system.

Option B: View System Information window

1. Click Start, and then click Run.
2. Type winmsd.exe, and then click OK.
3. Select the System Summary in the navigation pane.
4. Locate Processor under Item in the details pane.
   • If Processor value starts with x86, it is the 32-bit system.
     
   • If Processor value starts with ia64 or AMD64,it is the 64-bit system.
     

Windows Server 2003

Option A: View System Properties in Control Panel

1. Click Start, and then click Run.
2. Type sysdm.cpl, and then click OK.
3. Click the General tab. Under System, if you see ;Microsoft Windows Server 2003 Enterprise x64 Edition you have a 64-bit system.  If you see only Microsoft Windows Server 2003 Enterprise Edition it is the 32-bit system.

Option B: View System Information window

1. Click Start, and then click Run.
2. Type winmsd.exe, and then click OK.
3. Select the System Summary in the navigation pane.
4. Locate Processor under Item in the details pane.
   • If Processor value starts with x86, it is the 32-bit system.
     
   • If Processor value starts with ia64 or AMD64,it is the 64-bit system.