There's a fix for that!


Whether caused by a virus, a new operating system or by simple mistake, being faced with an "Operating System not found" or similar error during your computer system's boot up can be a nerve rattling experience. Assuming you have Microsoft's Window 7 installed however, do not fear, such a boot error can often be resolved in just a few simple steps.


First, check your BIOS and hardware

In many cases, having Windows 7 fail to boot may be as simple as having your BIOS set with an incorrect boot order sequence. It can be quite common if you have more than one hard drive installed in your computer and your BIOS gets reset. Usually you can access your BIOS seconds after your computer turns on by pressing the Delete button or by pressing a specific function key. Once in the BIOS, check to see that your system drive is listed appropriately in the boot order sequence; you may need to refer to your motherboard manual for help.

Another possible reason for Windows not being detected upon start up is a hardware issue. If your BIOS is unable to detect your system drive, check to make sure all the cables are plugged in properly. If your hard drive is making an odd noise, such as a clicking sound, your hard drive may be broken. Finally, it is possible that the hard drive is having data corruption issues, which has damaged important system data, such as the Master Boot Record (MBR). If you suspect a faulty hard drive, it may be a good idea to backup and scan your hard drive for errors from another computer and possibly consider buying a replacement. Trying to repair a boot problem on a damaged drive can possibly lead to even more data loss, so backup your data before attempting anything.

The MBR and other important boot data can also be damaged by trying to install an earlier version of Windows, such as Windows XP, alongside Windows 7 and by third-party programs, such as viruses. In the case of a virus, it is recommended that you run a virus scan of the drive before attempting any repairs as otherwise it could lead to more data loss. It is further possible to achieve the appearance of damaged boot data by having the wrong drive partition set to active, which can be the outcome of an overly curious Windows user with administrative permissions.

Fixing the MBR and other start up problems in Windows 7 is most quickly accomplished by using the Windows 7 Installation DVD. If you do not have a Windows 7 Installation DVD however, you can alternatively use a Windows 7 System Recovery Disc, which we will show you how to create further down in this article.  If you do not yet have either a Windows 7 Installation DVD or a recovery disc, do yourself a big favor and make a recovery disc right away to avoid any unnecessary headaches down the road.

Fixing the Master Boot Record (MBR)

Step one: Turn your computer on, booting from either your Windows 7 Installation DVD or Windows 7 System Recovery Disc.  Remember, you may need to change the boot order inside your BIOS to have the your DVD drive boot first.

Step two: After the installation or recovery disc loads, if prompted, select your language settings and then continue.  If you are using the installation DVD, when prompted by the following screen select Repair your computer.

Zoom
Step three: The computer will take a moment now to scan itself for any Windows installations, after which you will likely be given a choice to select which installation you wish to repair.  Select the appropriate Windows installation from the list and then continue. If by chance a problem is detected in one of your Windows installations at this initial stage, the system may also ask you if it can try to repair the problem automatically. It is up to you if you wish to let the system try to repair itself, but otherwise just select No. 

Step four: Once you have reached the System Recovery Options screen, as shown below, you will be faced with a list of choices that can aid you in repairing a damaged Windows 7 operating system.  If you wish to try the Startup Repair option first, it is often successful in automatically fixing many different start up issues, but in this article we will be using the Command Prompt option to resolve our problems manually. So, click Command Prompt to continue. 

ZoomStep five: Now sitting at the command prompt, enter the following command and then press enter:

         bootrec.exe /FixMbrIf
successful, you should be greeted with the message The operation completed successfully.  That's it!  Your Master Boot Record has been repaired.

While the above command does fix the MBR, and sometimes that is enough, there still might be an error with the system partition's boot sector and Boot Configuration Data (BCD). This might occur if you have tried to install another operating system alongside Windows 7, such as Windows XP.  To write a new boot sector, try the following command:

        bootrec.exe /FixBoot If you are still faced with your Windows 7 installation not being detected during start up, or if you wish to include more than one operating system choice to your system's boot list, you can try the following command to rebuild your BCD:
       
        bootrec.exe /RebuildBcd

The above command will scan all your disks for other operating systems compatible with Windows 7 and allow you to add them to your system's boot list. If this fails, you may need to backup the old BCD folder* and create a new one in its place with the following commands:

        bcdedit /export C:\BCD_Backup
        c:
        cd boot
        attrib bcd -s -h -r
        ren c:\boot\bcd bcd.old

        bootrec /RebuildBcd
*Some users also find simply deleting the boot folder and retrying the above steps effective at resolving boot issues, but it is not recommended.

How to change active partitionsUpon purposely changing the active partition on my system drive, I was faced with a BOOTMGR is missing error during my system's start up that prevent Windows from starting. It is a common mistake to make when playing with partitions on a system drive and it can be a headache to solve if not prepared. To change your active partition back using the Windows 7 recovery disc or Installation DVD, follow the steps below.

Step one: Follow steps one to four in the above guide. This should take you to the Command Prompt in the Windows Recovery Environment.

Step two: Type DiskPart and then press Enter.

Step three: Type List Disk now and then press Enter. This command will list all disks attached to your computer and assign them a disk number.

Step four: Type Select Disk x, where x is the number for the disk containing the partition you wish to make active. Press Enter.

Step five: Type List Partition and then press Enter. You will now be shown a list of the partitions on the selected disk. Determine which partition you wish to make active.

Step six: Type Select Partition x, where x is the number of the partition you wish to make active.

Step seven: Now, just type Active and then press Enter. That should be it - the selected partition is now active.

ZoomHow to create a Windows 7 System Recovery DiscWindows 7 makes it easy to create a System Recovery Disc if you already have Windows 7 installed and running. 

Step one: Click Start > All Programs > Maintenance > Create a System Repair Disc

Step two: Insert a blank CD or DVD into your disc drive.


Step three: Click Create disc and let the program do its thing.

Zoom

That's it! It only needs to write about 140- to 160-megabytes to the disc, depending on whether your OS is 64-bit or 32-bit, and that should only take a minute. If you do not have a CD/DVD-R drive to create a recovery disc with, you can alternatively download the ISO image of the Windows 7 System Recovery Disc and use it to make a bootable USB flash drive.


How to create a Windows 7 System Recovery USB flash driveStep one: If you do not have a DVD drive, download the appropriate Windows 7 Recovery Disc image from Here. You will need a torrent client for this step, such as µTorrent. Alternatively, if you have a DVD drive, you can use an existing Windows 7 Installation DVD or a Windows 7 Recovery Disc when at step seven. 

Using a Windows 7 Installation DVD at step seven will also allow you to install Windows 7 via USB, not just recover a damaged system; very useful if you have a netbook!

Step two: Open a command prompt with administrative rights. To do this, click Start > All Programs > Accessories and then right click Command Prompt, followed by clicking Run as administrator.

Step three: After accepting any UAC verification questions, you should now be at the command prompt. Make sure your USB flash drive is plugged in and then type DiskPart, followed by pressing Enter.

Step four: Type List Disk and then press Enter. Determine which disk number corresponds to your USB flash drive. In the following scenario, Disk 1 corresponds to our USB drive since we know our USB drive has a capacity of 2-gigabytes.

Step five: Enter the following commands in order, changing the disk number to the disk number listed for your USB drive.  Warning - the following commands will erase everything on your USB drive or the disk you select.

        Select Disk 1

        Clean

        Create Partition Primary

        Select Partition 1

        Active

        Format FS=NTFS

Step six: After DiskPart successfully formats the USB drive, which might take a few minutes, you will want to enter the following commands:

        Assign
        Exit

Zoom

Step seven: You will now need to copy the contents of the ISO image you downloaded, or the contents of a DVD you wish use, to the USB flash drive.  There should be two folders and a file in the ISO image that need to be copied. To extract the files contained within an ISO image, you will need to use a program such as WinRAR. You can download WinRAR from here.


Step eight: Now that the files are copied, we will want to make the USB drive bootable. To accomplish this however we will need to download a small file called bootsect.exe; it can be downloaded from here or here. The file can also be found in the boot directory of the Windows 7 Installation DVD.  Once downloaded, place the bootsect.exe file in the root directory of your USB flash drive.

Step nine: Back at the command prompt, we will want to change the current directory to that of the USB drive and run the bootsect command. In our case this is drive E, so we will be using the following respective commands:

          e:
          bootsect /nt60 e:
The bootsect command will update the target volume with a compatible bootcode. If all goes well, you should now have a bootable USB recovery drive; just remember to add the USB drive to the boot list in your system's BIOS for it to work upon start up.

Reference: http://www.tomshardware.com/news/win7-windows-7-mbr,10036.html