Fix BOOTMGR Missing Errors in Windows 7, 8, and Vista

Perform the following methods to resolve the issue:
Method 1: Startup Repair from the Windows Recovery Environment (WinRE)
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1. Insert the Windows 7 installation disc into the disc drive, and then start the computer.
2. Press a key when the message indicating "Press any key to boot from CD or DVD …". appears.
3. Select a language, a time and currency, and a keyboard or input method, and then click Next.
4. Click Repair your computer.
5. In the System Recovery Options dialog box, choose the drive of your Windows installation and click Next.
6. At the System Recovery Options Dialog Box, click on Repair your computer.
7. Click the operating system that you want to repair, and then click Next.
8. In the System Recovery Options dialog box, click Startup Repair.

Refer the following link for more information:
http://windows.microsoft.com/en-US/windows7/What-are-the-system-recovery-options-in-Windows-7
To know more about “Startup Repair” visit the following link:
http://windows.microsoft.com/en-us/windows7/Startup-Repair-frequently-asked-questions

Method 2: Rebuild BCD using the Bootrec.exe tool
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To run the Bootrec.exe tool, you must start Windows RE. To do this, follow these steps:
1. Put the Windows Vista or Windows 7 installation disc in the disc drive, and then start the computer.

2. Press a key when you are prompted. Select a language, a time, a currency, a keyboard or an input method, and then click Next.

3. Click Repair your computer.

4. Click the operating system that you want to repair, and then click Next.

5. In the System Recovery Options dialog box, click Command Prompt.

6. Type Bootrec.exe, and then press ENTER.

Source: answers.microsoft.com