Check the platform installed on your Windows XP computer

Method 1

Step 1: Click Start, click Run, type sysdm.cpl, and then click OK.
Step 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 <> appears under System.
• For a 32-bit version operating system: Microsoft Windows XP Professional Version<> appears under System.
Note is a placeholder for a year.

Method 2

Step 1: Click Start, click Run, type winmsd.exe, and then click OK.
Step 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.

For more information, I would like you to follow the below Microsoft KB article:
http://support.microsoft.com/kb/827218