If there is a problem with your Master Boot Record then you will not be able to boot your system properly. MBR can be affected by many things such as corrupted by disk errors, it maybe also overwritten by other boot records, malware and viruses. After the MBR is corrupted you find your disk suitable only to store backup copies? Here is How to Recover or Fix Master Boot Record (MBR) in Winodws XP :
1. Insert Windows XP Installation CD and Boot the System.
2. Now Press 'R' when you are asked to repair a Windows XP installation.
3. Now to fix MBR use 'fixmbr' command.
4. Press 'Y' and fix the MBR.
5.Once done type in 'Exit' to exit the Recovery Console and Reboot.
This will fix the errors on your master boot record and you can boot your computer with this disk.