Have you tried to boot in Safe mode? If not, please, try it and tell me if you get any type of error.
Firstly, I think you could restore the operating system through a repair CD, if you have one.
Restart your computer, go after pressing the appropriate F key to navigate to the Windows menu. From there you should have access to restore your OS. Open the Command Prompt in safe mode and run the command:
sfc / scannow
Then press Enter and wait for your OS to be repaired. If you cannot access this menu while you reboot, enter the BIOS settings.
Secondly, you can navigate to the storage menu in the BIOS to verify that your hard drive is set at the first boot priority. If not, you have to manually set it.
Once you have done this please, come over here to tell me what happened.