A Master Boot Record [MBR] is used for bootstrapping OS.If, for any reason, the MBR becomes damaged or corrupt, then your computer will not start. You will normally see error messages like:

*  Error loading operating system
*  Invalid partition table etc.

These messages take breath out of most us (even mine as it wastes my time :@). Most people would automatically assume their computers are dead! But, that’s not true at all .

It’s actually easy to fix the Master Boot Record in XP ,Vista and Win 7

Howto repair your damaged MBR?

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1)Fix MBR in Windows XP
The steps to follow :

1. First, restart your computer with the Windows XP setup disk in the CD drive.

2. When prompted, boot from the CD drive by pressing any key. If Windows loads automatically, you will first have to enter the BIOS setup and change the order of the boot devices to start with the CD drive.

3. Once the setup loads, you will see the option to press R to repair a Windows installation.


4. Once the Recovery Console loads up, you will have to type in a number that corresponds to your Windows installation. This is normally just 1. Press Enter and then type in the Administrator password.


5 .Inside the console, type the following to fix mbr :
    bootcfg /rebuild (to rebuild your boot config)
    fdisk /mbr 
    Shutdown
   fixmbr
   fixboot 
NOTE : make sure you only use these commands on a system with one operating system installed. If you have more than one operating system installed, fixmbr and fixboot could mess up everything.

2)Fix MBR in Windows Vista  

In Vista, the procedure to fix the master boot record is a bit different. You have to start up Vista in the Recovery Environment and then run the bootrec command. Here’s how.
       1. First, load up the Windows Vista disc in your drive and press any key to boot from the disc.

       2. Choose the language, time, currency, etc and click Next. Now click on Repair Your Computer.
       3. Choose the operating system to repair and click Next. When the System Recovery Options dialog comes up, choose the Command Prompt.

4. Now type bootrec.exe and press Enter.
   
5. After that just t ype in the following :
   bootrec.exe /fixmbr
   bootrec.exe /fixboot
   bootrec.exe /rebuildbcd
   
6. 
    Restart your computer now after removing the vista DVD

1. Insert Win 7 installation DVD and boot from DVD drive. While in some older systems you may have to change boot order through system BIOS, most newer systems allow booting from DVD without changing boot order by simply clicking on any key when prompted to doing so.
   
   
   
   
   
   
   
   
   
   
   

     2.  Choose your default "Language", "Time", and "keyboard Input" on the first window and click next.

3. You’re now presented with 3 choices. Click on "Repair Your Computer" to gain access to the System Recovery window.
   
   
   
   
   
   
   
   
   
   
   
   Now choose "Command Prompt" in order to run the desired utility which is called "bootsect.exe". Bootsect is located inside the boot folder so change your directory to boot.
   Now run
   "bootrec.exe /nt60 C:\" (default position of your Windows partition is C:/).
   After that, type
   bootrec.exe /fixmbr
   bootrec.exe /fixboot
   bootrec.exe /rebuildbcd