How to check whether the OS installed is 32-bit or 64-bit

Method 1

1. Click Start, then click on Run or Start Search.
2. Type msinfo32.exe and then press Enter key.
3. In “System Information”, review the value for the System Type item:
   * For 32-bit editions of Windows, the value of the System Type item
     is x86-based PC.
   * For 64-bit editions of Windows, the value of the System Type item 
     is x64-based PC.

Method 2

1. Click Start, click Run, type sysdm.cpl, and then click OK.
2. Click the General tab. The operating system appears as follows:
   * For a 64-bit version operating system: Microsoft Windows XP 
     Professional x64 Edition Version <Year> appears under System.
   * For a 32-bit version operating system: Microsoft Windows XP 
     Professional Version <Year> appears under System.

Method 3

1. Click Start, click Run, type winmsd.exe, and then click OK.
2. In the details pane, locate Processor under Item. 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.

reference:
how to check if windows xp system is 32 bit or 64 bit