Malware, viruses and installing secondary operating systems on the same PC are all reasons you might receive a missing MBR or Bootmgr warning message when starting your PC. In most cases, repairing a missing MBR or Bootmgr.exe message requires you insert the original Windows disc and perform a repair installation of the operating system. If you have lost your original Windows disc or it becomes damaged, repairing the corrupted boot files might seem impossible. Nevertheless, if you have access to another PC with Internet access, you can download and burn a live version of Linux you can use to get your Windows PC running again quickly. Even though Linux is a completely different operating system from Windows, it can help you fix your Windows OS installation in this way.
Select and download a Live Linux distribution – commonly referred to as a "distro." Knoppix and Ubuntu Linux are commonly used distros for repairing Windows installations. However, just about any distribution of Linux is suitable for repairing MBR or bootmgr.exe errors. You can view many Linux distributions at sites such as Livecdlist.com and Distrowatch.com. Save the ISO image for the Linux distro to a folder on your computer. Note that depending on the size of the Linux distro you select, the download may take from several minutes to a couple of hours to complete.
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Insert a blank CD into the optical burner drive of the computer. If the Windows AutoPlay box appears, click the "X" in the top right-hand corner of the window to close it.
Launch the CD burner application on the computer. Click the "Burn ISO" or "Burn Image" option on the menu or toolbar. Select the ISO image file for the Linux distribution you downloaded earlier. Follow the prompts to burn the image to the blank disc. Remove the newly created Live Linux CD from the burner when prompted.
Insert the Live Linux CD into the optical drive with the faulty MBR or bootmgr.exe file. Power on the computer and press a key on the keyboard when prompted to boot the PC from the Live Linux CD. Wait a few minutes for Linux to load from the CD and the desktop screen to appear on the monitor.
Click the "Start" or "Launch" button on the Linux taskbar or dock menu. Click "Accessories," then "Terminal." A command prompt window appears on the monitor.
Type the following command at the Linux command prompt: sudo apt-get install syslinux Press the "Enter" key. Wait a few minutes for the machine to connect to the package repository site for that particular distribution and download the package needed to repair the MBR or bootmgr.exe error. A message should appear confirming the successful download of the package. Note that if "sudo apt-get install syslinux" returns an error, try "sudo apt-get install mbr" instead. The "syslinux" package may not be available for all Linux distros.
Type the following command at the command prompt if you used the "syslinux" package: sudo dd if=/usr/lib/syslinux/mbr.bin of=/dev/sda Press the "Enter" key and wait for the package to repair the corrupted MBR record or bootmgr.exe file.
Type the following command at the command prompt if you used the "MBR" package: sudo install-mbr –I n –p D –t 0 /dev/sda (be sure to include the spaces) Press the "Enter" key and wait for the package to make the needed repairs.
Remove the Linux Live CD from the optical drive on the PC and restart the computer. After the PC restarts, the machine boots normally into Windows.