first get intouch who ever you bought the computer and DEMAND either a restore disc or the full XP disc. you can take your drive out set it as slave with the jumpers at the back of the drive and put it in another computer to save all you data then you can reinstall windows when you get a disc (hopefully of who you bought the com of)
also you should download
THIS run it and it will give you the product key for your windows xp write it down keep it safe then all you need to do is either get a friend to copy xp home for you and install it on your computer which is all legal because you are using your own product key.