From support.microsoft.com Article ID: 927392
You can use the Bootrec.exe tool in the Windows Recovery Environment (Windows RE) to troubleshoot and repair the following items in Windows Vista or Windows 7:
A master boot record (MBR)
A boot sector
A Boot Configuration Data (BCD) store
Note When you are troubleshooting startup issues by using the Windows RE, you should first try the Startup Repair option in the System Recovery Options dialog box. If the Startup Repair option does not resolve the issue, or if you must troubleshoot more steps manually, use the Bootrec.exe tool.
To run the Bootrec.exe tool, you must start Windows RE. To do this, follow these steps:
Put the Windows Vista or Windows 7 installation disc in the disc drive, and then start the computer.
Press a key when you are prompted.
Select a language, a time, a currency, a keyboard or an input method, and then click Next.
Click Repair your computer.
Click the operating system that you want to repair, and then click Next.
In the System Recovery Options dialog box, click Command Prompt.
Type Bootrec.exe, and then press ENTER.
Note To start the computer from the Windows Vista or Windows 7 DVD, the computer must be configured to start from the DVD drive. For more information about how to configure the computer to start from the DVD drive, see the documentation that is included with the computer or contact the computer manufacturer.
The Bootrec.exe tool supports the following options. Use the option that is appropriate for your situation.
Note Use the Bootrec.exe tool to troubleshoot “Bootmgr Is Missing” issue. If rebuilding the BCD does not resolve the startup issue, you can export and delete the BCD, and then run this option again. By doing this, you make sure that the BCD is completely rebuilt.
To do this, type the following commands at the Windows RE command prompt:
bcdedit /export C:\BCD_Backup
attrib bcd -s -h -r
ren c:\boot\bcd bcd.old
The /FixMbr option writes a Windows 7 or Windows Vista-compatible MBR to the system partition. This option does not overwrite the existing partition table. Use this option when you must resolve MBR corruption issues, or when you have to remove nonstandard code from the MBR.
The /FixBoot option writes a new boot sector to the system partition by using a boot sector that is compatible with Windows Vista or Windows 7. Use this option if one of the following conditions is true:
The boot sector was replaced with a nonstandard Windows Vista or Windows 7 boot sector.
The boot sector is damaged.
An earlier Windows operating system was installed after Windows Vista or Windows 7 was installed. In this scenario, the computer starts by using Windows NT Loader (NTLDR) instead of Windows Boot Manager (Bootmgr.exe).
The /ScanOs option scans all disks for installations that are compatible with Windows Vista or Windows 7. Additionally, this option displays the entries that are currently not in the BCD store. Use this option when there are Windows Vista or Windows 7 installations that the Boot Manager menu does not list.
The /RebuildBcd option scans all disks for installations that are compatible with Windows Vista or Windows 7. Additionally, this option lets you select the installations that you want to add to the BCD store. Use this option when you must completely rebuild the BCD. WAZOO
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