On Thu, 26 Mar 2020 07:52:56 -0000 (UTC), Arlen Holder wrote:

I find it hard to believe we zeroed in so easily on the culprit, but I'll
keep the PC alive for another day just to make sure, before running full
memtest86 tests (as suggested by n/a & Paul) on all four memory sticks.

UPDATE:

The PC stayed alive until earlier this week, with zero problems.
o That's with the RAM cards all back in the original four slots

Then, I put the Nvidia GeForce 210 graphics card back...
o And, it worked fine, for, oh, a few hours.

Albeit, the Nvidia GPU fan was super noisy the whole time...
o Then... suddenly.... wham!

Now the PC won't reboot again... for days.
o So I'm starting over... (without the Nvidia card!)

Interestingly, the boot won't recognize the HDD at all...
o I get the Windows blue flag on a black background... forever.

I'm booting to a flash drive as we speak...
o And trying to figure out (on another PC) what the commands are...

So far, I've tried (to no avail)...
o bootrec.exe /fixmbr
(that works)
o bootrec.exe /fix boot
(access is denied)
o bootrec.exe /scanos
(total identified windwos installations = 0)
etc.
--
In addition, the BIOS screen has strange vertical lines in it, as if the
graphics card on the motherboard has been, somehow, affected.