This is necessary when the partition boot sector has become corrupt due to a virus or some kind of damage or is unstable thanks to configuration problems.
Follow these easy steps to repair a damaged or unstable partition boot sector in Windows XP.
Enter Windows XP Recovery Console. The Recovery Console is an advanced diagnostic mode of Windows XP with special tools that will allow you to write a new partition boot sector to a your Windows XP system partition.
The fixboot utility will write a new partition boot sector to the current system partition. This will repair any corruption that the partition boot sector may have and will undo any partition boot sector configurations that may be causing your problems.
Take out the Windows XP CD, type exit and then press Enter to restart your PC.
Assuming that a corrupt or unstable partition boot sector was your only issue, Windows XP should now start normally.