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#21
Just my two cents and forgive me if this was said as I haven't read the whole thread but...
Operating system not found is generally code word for your HD is going bad or some type of hardware problem that is not allowing your computer to recognize your hard drive. At this point in the boot process, windows has just begun to point to the operating system but wasn't able to since it couldn't find the hard drive or the sector containing the MBR.
I would definitely troubleshoot to see if hardware failure is the cause.