if its a legal version that you have bought and paid for contact microsoft and they will supply you with a legitimate key. if its an illegal version your trying to activate then phone up microsoft and go buy yourself a real software license
when you try to activate windows xp and it does not take there should be an option to phone MS. here is windows xp home for $59.00 , Most of these are store fronts. I have purchesed sftwr from them with good results. I am in no way endorsing them. You have to read the posts of other purchasers . Most of these are OEM sftwr, meaning you get just the cd and cd key. http://www.pricegrabber.com/search_attrib.php?page_id=186&form_keyword=windows+xp+home&rd=1