"Operating System Not Found " - it means what is says: the system can't find an operating system to boot.
This can be caused by:
- Changes to the partition flags
- Corruption of the Windows boot files
- Corruption of the MBR or GPT
- Loose or poorly connected hard drive
- Failed hard drive
The first thing I would try is to use a bootable CD of Partition Wizard Partition Wizard Bootable CD allows user to manage partition directly with partition manager bootable CD.
and boot into it and see if you need to set the Windows partition to ACTIVE. If there is a 100MB System Reserved partition make sure that is Active. If there is not, make sure that C: is Active.
You will also be able to see if PW can find the drive at all.
If PW can see the drive and the correct partition is ACTIVE and the problem persists then the next thing I would do is try a Startup Repair: Startup Repair
You may need to run it 3 times: Startup Repair - Run 3 Separate Times
Hope that helps