XP usually creates auto restore points when you make significant changes so you need to find a point dated before the problem cropped up.
You need to check thoroughly for virus invasion and for spy programmes.
If you have the necessary Anti Virus and Anti Spy applications installed then you should find the offender(s)
If not there are free downloads available such as AVG Antivirus and Ad-Aware SE (both excellent). Simply enter these titles into your search engine via Internet Explorer. Well worth a try. Good luck