Windows 10 Pro will not run properly on new loading?

Hi, doesn't quite sound as if you've ever clean installed Win 10 in the normal way.

As you have (presumably) a validly activated Win 10 license, activation should now be automatic.

Create a bootable medium to clean install it - using another PC if yours won't boot.
Create Bootable USB Flash Drive to Install Windows 10

Then perform a clean install:
Clean Install Windows 10

Note: a common mistake is to not understand that a clean install means deleting existing partitions comprising your current Windows installation (4 in the case of a typical UEFI installation, fewer for MBR), and installing to unallocated space.
(There is a variant of this for advanced users)

This should give you the latest just released build 2004; some minor issues have been reported.

Once you have that running, then as early as you can start routinely using disk imaging to consolidate that and give you a means of recovery. E.g. Macrium Reflect (free) + large enough external storage for image files.

easier updates of software

I am running Linux Mint 19.3 [based on Ubuntu] and find its Software Updater for programs and Update Manager for Linux to work quite nicely, just a few prompts to accept and go on. In some ways it is better than Windows in not being automatic, do it when I want, or not. But just like with Macintosh there's times when things are better with Windows.

In your situation I'd prefer having only one drive in the computer and booting to either the GPARTED LiveCD or Linux LiveDVD and wipe the drive completely then starting over with the Win10 install. The bootable Linux discs usually also include GPARTED [which runs a limited version of Linux]. They also have an app that can be used to create a bootable USB drive if the computer does not have an ODD.

I don't dual-boot, have a rehabbed Desktop and a Notebook with Linux, have no cost in them as I reuse parts.

Rapidrob said:

Both times were a clean install on a new hard drive, one an HDD the other a SSD.

Thank you for your reply and clarifying that. It wasn't clear from your first post which sounded more as if you'd somehow bought a disk with Win 10 on it and put that in your PC.

Try running checks from a Win 10 live boot disk. If that crashes, you know there's something amiss.

I won't bother to list the checks you could do as I assume you've done them.
Disk, ram, stress tests... Driver Verifier.. inspected logs..