There may come a time when you need to reformat your hard drive and reinstall Windows XP. Normally this would require that you contact Microsoft in order to get your system reactivated but if you are reinstalling onto exactly the same computer with the same hardware configurartion then you can bypass this if you plan ahead.
What you need to do is to save the file from your system that records the fact that the system is already activated and reload that file after reinstalling Windows XP. The file that you need to copy is C:\windows\system32\wpa.dbl and you can use whatever method you like to make a copy of this file as long as the copy isn't stored on the hard drive that you are about to reformat to do the reinstall. As a safety precaution you should also take a copy of the wpa.bak file as well.
The next step is to reinstall Windows XP. When the install process gets to the point where it asks you if you want to activate your system you tell it no.
After reinstalling XP boot your computer into safe mode so that you can copy the file(s) that you copied earlier back to their original location (if you don't go into safe mode the files will be in use and you wont be able to overwrite them). Again as a safety precaution you should consider renaming the new versions of the files rather than just overwriting them.
This article written by Stephen Chapman, Felgall Pty Ltd.