I do not condone this. I bought and paid for two versions of WinXP Home and have them installed on our two XP Home systems. 100% Legit.
That being said, a lot of you have pirated copies. And because your are "pirates", a lot of you cannot install the critical SERVICE PACK 1. These instructions from a knowledgeable yet anonymous source should go a long way in helping you.
Best of luck!
UPDATE 7/18/2006 – If you are trying to find the Product Key from your Windows XP CD, here's some info that might help.