First off, if it's a branded readymade PC, what make & model is it?
Can you possibly borrow a genuine WinXP Home CD from a neighbour/friend/relative? You can reinstall with the help of this.
Before you do anything, make sure you have the product key to hand. This should be present on a sticker on the case. If not, use MJB Keyfinder click here / Belarc Advisor click here to get this info from your system registry & make an accurate written note of it. Without this you will not be able to reinstall XP.
Before you reinstall afresh, don't forget to do a thorough backup of your emails, data, music, videos, etc. to another hard drive or external blank media (CDs/DVDs).
Once you are able to get all the above sorted, we can give you a step-by-step guide to reinstalling afresh. G