If you are installing Windows Xp Home, or XP professional, or Xp Media Center, it will be 32 Bit.
If you are installing XP X64 Professional it will be 64 bit.
This is considering the system you are trying to install, not what you already have.
This is the driver Setup is looking for when it says to press the F6 key to install 3rd party drivers shortly after starting your process to make a new install. This driver will be merged into Windows permanently.