Booting into Windows gives "Operating System not found"
Try to repair the Windows boot sector using the Startup Repair Tool by Microsoft.
You can attempt a Startup Repair using these instructions:


Insert the Windows 7 installation disc or USB flash drive, or a system repair disc, and then shut down your computer.

Restart your computer using the computer's power button.

When prompted, press any key, and then follow the instructions that
appear.

On the Install Windows page, or on the System Recovery Options page,
choose your language and other preferences, and then click Next.

If you are using the Windows installation disc or USB flash drive,
click Repair your computer.

Select the Windows installation you want to repair, and then click
Next.

On the System Recovery Options menu, click Startup Repair.