My system is running WIndows 8.1 *Home*, not *Pro*, and yet my attempts to upgrade to Windows 10 fail at exactly the same "73%".
I have run Windows Update until there are no more updates, have more than enough storage space on the hard drive, have memory that passes all tests, have a CPU that is more than fast enough for Windows 10, and I am trying to upgrade from a DVD with the latest ISO burned to it to upgrade. No matter what I try, all attempts fail at "73%". It seems to me that this is consistent for enough of us in the Microsoft Windows community that someone at Microsoft should be able to tell us what to look for in our systems to try to get past this.