You might want to try logging on as the administrator account and changing the password that way. If you boot up in safe mode (standard way is F8 while the computer boots up) then you may notice another user account called administrator that is usually hidden. Depending on your XP version the password of this may still be a default one. Possibly either "administrator" or nothing. If you can get into this then Start->Control Panel->User Accounts (cant remember the exact name i'm on a network pc so can't check it) will allow you to modify passwords.
Like vehelpfulguy suggested though, if you just bought the pc, it's unlikely there's anything on it, so a reformat and reinstall of windows is a good option.