How to Manually Repair Windows 7 Boot Loader Problems

The first thing you’ll need to do is boot off the install disc, and then click through until you see the “Repair your computer” link in the lower left-hand corner.

You’ll need to choose the correct installation of Windows and then click the Next button.

And then you’ll get to the System Recovery Options screen, where you can get to the Command Prompt. 

Repairing the Master Boot Record

If you want to restore the master boot record, you can simply type in the following command:
bootrec /fixmbr
You can also write a new boot sector onto the system partition with this command (which is often more useful):
bootrec /fixboot
And of course, if you just use bootrec /? you’ll be able to see all the options.