If both instances of xp pro were installed on the same partition
then take a look here:
http://support.microsoft.com/?kbid=888023
Basically what it comes down to is you have to boot into the
instance of XP pro that you want to keep. You then identify which
windows folder to keep and delete the other one.
The next step is to edit your boot.ini file to reflect this
change.
An alternative approach to edit the boot.ini file is to go to
Start -> Run and type msconfig in the Open box. Click on the
Boot.ini tab. Click on "Check all boot paths". Make sure that the
entry in the box that pops up points to the installation you're
trying to remove. If it is select yes and close the System
configuration utility. Select the restart option. When you're PC
reboots a box will be displayed informing you that the Windows is
now running in diagnostic mode. Put a check mark in the box so
that the message is not displayed again and close the box.