If you were to reinstall Windows XP, you normally must re-activate it as well, even if your hardware is unchanged... luckily, there's a way around this.
First, your hardware between reinstalls must NOT change..
XP keeps the activation information in a file named "wpa.dbl", which can be found in _:\WINDOWS\System32\
After your activating Windows XP, back this file up to a floppy or some other type of media.
Now, if you ever need (or want) to reinstall Windows XP, go through the whole procedure as you normally would. Once you get into Windows itself, copy the file from your media back into _:\WINDOWS\System32\... You may need to be in safe mode to do this, however.