I would recomend that unless it is a new computer or you recently formated the hard drive that you go to the malware link in my signature and follow those steps.
Also i would download or buy an virus scanner (like avg, or mcafee (free with aol),
http://www.pandasecurity.com is an online scanner, with others
Also i like Spybot S&D and Ad-ware but there are others also.
Also i would get a firewall like ZoneAlarm since this will also help and it is more effective then wht comes with XP.