Hello and welcome to Tom's Hardware Forums.
XP Home is a product which has to be bought and paid for and when you do that, you will get a Product Key and be able to activate the legitimate system. If your computer has a sticker on it - the Certificate of Authenticity - with a Product Key printer on it, that has the match the key in the system. Google for and download Magic Jellybean Finder and use it to find that system key and if it doesn't match, your system is illigitimate.