Having spent far more of my time fixing Windows problems than I really wish to recall :-/, what I've found to work most reliably in practice is to wipe the harddisk and reinstall the operating system.
On the one hand, this will take a day of work (either of your time, or, if you don't have the expertise to do it yourself, of someone you pay to do it). On the other hand, you'll usually end up with a working computer; and the process is usually merely tedious.
On the third hand, naturally you may find a less drastic solution to this particular problem; on the fourth hand, it might take you as long or longer to find that fix than to do the full reinstall.
I don't know enough about Macs to give advice as to whether that would be a better solution for you.