Changing XP license key

I've done this once before, but now can't quickly find the information. Hopefully someone will be able to fire off a quick reply or I might have to do some work...

I'm looking to change the license key in an installed XP. The system is currently running with a legal, valid key, but for various administrative reasons I need to assign a different license (valid, purchased) key to that system and re-use the license key on a different system (delete long complicated bureaucratic reasons why I need to do this).

So my questions are:
1) What program can I use to change the license key in a running version of XP?
2) What program can I use to discover the current license key of a running system, given that someone's cat has used the license sticker as a scratch and chew toy?

 

Download Belarc Advisor, (Google it) run it and after a few minutes a screen will appear showing a huge amount of info about what your computer contains. Scroll down and a you show see the license key numbers you need.

 

OK, thanks you two. Any advice on how to change said key?

 

Try these two free programs:
www.magicaljellybean.com/keyfinder.shtml
www.winkeyfinder.com