You will need to remove the 64 bit version first and then install the 32 bit
version. You can't simply install a 32 bit version over top of a 64 bit
version. This means you will be starting fresh and will be wiping what is
currently on the drive. So if you have anything to backup ensure you do it
before you do the new install.
To run the fresh install boot the computer with the new 32 bit CD. Restart
the computer and press the Pause/Break key on the keyboard. That will stop
the computer from continuing until you press the Enter key. So with the
system paused insert your new XP Pro 32 bit CD, close the tray and press the
Enter Key. In a moment you should see a message that says "press any key to
boot cd..." You have a 5 seconds to press a key. You should see an blue
screen with info that Windows is setting up some files. If your computer has
a SATA drive you will need to press the F6 key when instructed to do so (at
the bottom of the screen) and then pop in the floppy or disk that came with
your computer that contains the drivers for your SATA drive. If you computer
doesn't have a SATA drive then do not press the F6 key just let the process
continue.
So you will get to another blue screen that gives the option to install
Windows or use the Recovery Console. Just press the Enter key and you will
be greeted with a screen asking you to accept the License Agreement. Press
the F8 key and the next screen will give you options to install. Select the
option to Delete partitions, Create a new partition, then Format as NTFS.
Once you have formatted the XP CD will continue to install XP. Typically it
goes very smoothly and takes about 45 minutes.
See here for detailed information including screen shots
http://www.petri.co.il/install_windows_xp_pro.htm