The reason that you are getting this error is because the Windows Boot Manager (Bootmgr) entry is not present in the Boot Configuration Data (BCD) store or that the Boot\BCD file on the active partition is damaged or missing.
That is why you are now getting the "NoOSInstalled under signature 07" message after trying the Start-up Repair option as suggested by tf76.
To rectify your problem you can rebuild the BCD store using the Bootrec.exe tool in the Windows Recovery Environment.
Do this by following these steps:
1. Re-start the computer, pressing the F8 key at the same time (Windows Recovery Console is part of Win 7 installation, no need to use your installation DVD).
2. Select "Repair Your Computer" from the screen that appears. Windows loads the necessary files.Click NEXT
3. Select a language and a keyboard on the screen that appears and then click Next.(Windows will now search for installed operating systems, if you only have one, Win 7, it will be selected otherwise Click the operating system that you want to repair).Click NEXT.
4. In the System Recovery Options dialog box, click Command Prompt.
5. Type Bootrec /RebuildBcd and ENTER.
6. If Bootrec.exe runs successfully, you will see the installation path of the Windows directory. To add the entry to the BCD store, type Yes. A confirmation message appears that indicates the entry was added successfully.
7. If Bootrec.exe does not find any missing Windows installations, you must remove the BCD store, and then you must re-create it.Do this by typing the following commands.
Bcdedit /export C:\BCD_Backup - press ENTER
ren c:\boot\bcd bcd.old - ENTER
Bootrec /rebuildbcd - ENTER
8. Restart the computer and you should be good to go.