When you said completed OC SUCCESFULLY, Did you test your Memory (ie use memtest86 from a Bootable CD) BEFORE restarting windows.
It is possible that your memory was not stable with the OC and this may have corrupeted the windows file(s).
Another possiblity - When you reset Bios Back to default - Did you recheck the BIOS setting for the HDD. This need to be the same as when windows was installed - and may NOT be the default value.
.. Look at what it is set to IDE, AHCI, or raid. If it is set to ide, then change to AHCI and retry, if it was set to ahci, then change to IDE and retry. Neither IDE or ahci work then try Raid.
Might be helful for others if You Identify your system, ie MB, Ram, CPU