Depends on which kind of Windows you have installed 32 bit or 64 bit.
Open Start Menu > Run
and type winver
and press Enter. It would open the About Windows
dialog. If you have 64 bit Windows, you would see 64 bit edition written somewhere in that dialog. If you dont see it, you have 32 bit version.
Besides when 64 bit Windows XP boots, the boot logo has BIG x64 edition
written under the Windows logo.
If in confusion, just download the setup version( http://www.cpuid.com/download/cpuz/cpuz_152_setup.exe
) that contains both 32 bit and 64 bit versions. The setup would install the correct version automatically.