from http://support.microsoft.com/kb/827218
Windows XP
If you have Windows XP, there are two methods to determine whether you are running a 32-bit or a 64-bit version. If one does not work, try the other.
Method 1: View System Properties in Control Panel
Click Start, and then click Run.
Type sysdm.cpl, and then click OK.
Click the General tab. The operating system is displayed as follows:
- For a 64-bit version operating system: Windows XP Professional x64 Edition Version (year) appears under System.
- For a 32-bit version operating system: Windows XP Professional Version (year) appears under System.
Method 2: View System Information window
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 Windows.
- If the value that corresponds to Processor starts with ia64 or AMD64, the computer is running a 64-bit version of Windows.
Program Files (x86)folder. If there is, that's 64-bit Windows. – Randolph West Jan 11 '10 at 9:33