If you have access to your Windows XP system (or a backup) you need to grab a copy of the activation files before you reformat and install a fresh copy.

The information about the current activation is stored in an encrypted database file named Wpa.dbl, which you can back up and then restore once you've reinstalled XP. Remember that this technique only allows you to reinstall XP on the same computer--if you've installed new hardware (like a hard drive) this may or may not work. That's the catch 22, no hardware changes.