Macron74,
If you bought OEM XP Pro, Carey's link should work.
If you bought any other OEM XP, you will have to back up your data and then do a clean installation.
In your case, it would have been much better from a personal security point of view to have done a clean, Genuine installation of XP right after you purchased the computer. Not doing so has left you potentially vulnerable to whatever malicious software may have been on the computer when you bought it.