have you removed or added in files in the system folder? could be the cause or maybe some malware affected your system if you have no security option on?
you need to format your windows, it seems some of the necessary system files needed to operate your windows have been stopped working correctly damaged or corrupted. can only fixed by formatting.
don't try to use a windows copy it might damage your system more. use an original windows disc with activation code, restart and follow your boot option you can see how to boot in boot black screen with options for some it's F8 others have another button to boot screen from there follow the guide, you can repair if possible if not just install new windows from your options.