The Master Boot Record in Windows 7 is what the operating system uses to boot from the hard drive. If MBR will be corrupted/damaged in any way of your PC. It could lead to problems with booting the computer and you will encounter the dreaded error message: “Operating System not found“. If this happens, a user needs to use the original Windows 7 installation disc and the "fixmbr" command to repair the operating system. You can fix this problem yourself, all you have to do is make sure you have the original Windows 7 Disc.
Steps to fix MBR in windows 7
1. Insert Windows 7 disc and reboot the computer. Press any key to continue when prompted.
2. Choose the Language, Time and Keyboard and then press Next. Usually, you do not have to change anything here.
3. Click the "Repair your computer" option.
4. Wait for the installer to scan for Windows operating systems on the computer.
5. Select "Windows 7" from the System Recovery Options window that appears and click "Next."
6. Select "Command Prompt" from the list of recovery options that appears.
7. Type "bootrec.exe /fixmbr" and press the "Enter" key. After the process completes, the computer reboots. Restart your computer and hopefully you will see the Windows logo instead of that horrible “Operating system not found” error message.
If you are facing any problem while performing this steps you may take help from windows 7 support at their Toll Free No. +1-866-541-1792