If you have to reinstall Windows XP on the same computer e.g. because your old installation went instable or you tested some software, you normally have to activate your copy of Windows XP again. This will work 2 or 3 times with the online acticvation but then Microsoft will force you to call them by phone to get a very long activation number. To avoid the activation procedure for the new installation, make a copy of the file:
on your current activated installation to a USB disk or external harddrive and copy it back to the new installation.