A powerful and flexible tool that comes with every Windows XP setup CD is the Recovery Console.
It proved really helpful in repairing a damaged partition boot sector in one Windows installation.
Recovery Console turns out to be useful, even when safe boot mode and other startup options do not work. This is certainly the case when the partition boot sector, containing Windows boot code, has been overwritten.
Restart the computer from Microsoft Windows setup CD. At the first prompt, start the Recovery Console by pressing R.