How do you find out if your computer is 32 bit or 64 bit in Windows?
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
1. Click Start, and then click Run. 2. Type sysdm.cpl, and then click OK. 3. 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. Note <Year> is a placeholder for a year.
Method 2: View System Information window
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 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.
This information has been taken from *support.microsoft.com/kb/827218
for vista follow above link.