Some of the Windows files are corrupted - possibly due to a virus - try a repair of Windows.
Boot from your XP CD, ignore the first option to repair, proceed as if installing XP. The existing installation will be detected and the option to repair it will be offered - accept this.
Once repaired I would copy all personal data to dvd/external hdd run antivirus and malwarebytes. If infections are found I would look to a format and re install of XP