Did the os-prober effectively run? What is the output of… 
grep OS_PROBER /etc/default/grub
…?
GRUB_DISABLE_OS_PROBER=false
the results of the update grub were:
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-5.10-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.10-x86_64.img
Found initrd fallback image: /boot/initramfs-5.10-x86_64-fallback.img
Found linux image: /boot/vmlinuz-5.4-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.4-x86_64.img
Found initrd fallback image: /boot/initramfs-5.4-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Windows Recovery Environment on /dev/sda1
Adding boot menu entry for UEFI Firmware Settings ...
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
the “Adding boot menu entry for UEFI Firmware settings…” is a new message (with the 5-14 update). I have not seen that message before. That is usually where the message “found Window 7 on /dev/sda2” would be.
That’s what I already suspected earlier. Your Manjaro is running in UEFI mode instead of in BIOS mode.
So, what you have to do now is boot into your UEFI settings and re-enable legacy BIOS support — it might be called “legacy support” or “compatibility support module”. Then, make sure your system drive is selected as a BIOS-bootable drive, not as a UEFI drive.
Store the settings, and you should now be able to boot into Manjaro again. If that works, repeat the steps of…
- mounting the Windows partition; and…
- running
update-grub.
@Aragorn
Thank you for your time here. While I have nothing more important but this problem, I realize time is a precious commodity for (almost) everyone else.
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It’s okay. I still have a few hours before going to bed. I’d rather see this solved as well. 
You type too fast for me. You lost me at:
boot into your UEFI settings and re-enable legacy BIOS support
How do I boot into UEFI settings, or should I be searching the turorials again (and not bugging you)?
Well, the way you went into (what you called) the BIOS earlier. You reboot the machine and press the keyboard shortcut for entering the firmware settings.
My bios doesn’t support Uefi settings. I have Boot configuration: internal, external, or network: either enable or disable. External boot allows the first USB port to be used for booting. Internal allows the hard drive or the optical drive (DVD) to boot.
Yes, but there should normally be a setting pertaining to legacy boot or compatibility mode. There has to be, because your system was booted up in UEFI mode. That’s why instead of detecting Windows, it offered to add an entry for booting into the UEFI settings from within grub.
Maybe it’s in the boot order settings. In that case, you must set the legacy/compatibility mode first.
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@Aragorn
There is no legacy/compatibility mode. When this computer was new (2007)there was no legacy mode (or UEFI mode). It came loaded with Windows Vista. It IS a legacy computer. I loaded Manjaro because it had the best support for old hardware (according to my research) and the Cinnamon version because it was the best for beginners. It sounds as if my perfectly good laptop has hit the hardware limits of LINUX with Zetar version 25.
I don’t remember seeing the UEFI settings in the previous version when it finished an update. Is there a way to revert to version 24?
OR how do I coax Manjaro Zetar (25) to boot in legacy mode (not UEFI)? Surely, it is an option somewhere?
Let’s just make sure. Run this command again…
[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
[ -d /sys/firmware/efi ] && echo UEFI || echo BIOS
BIOS
I am genuinely stumped. 
Do you have the install-grub script installed? If you don’t, then install it first…
sudo pacman -S install-grub
It’s supposed to handle difficulties with grub by correctly reinstalling it. So after you’ve installed the package, try running it and see what gives…
sudo install-grub
If it doesn’t work, then I know whom to cast the blame on. 
@Aragon
Thank you. 
That is the first laugh I’ve had today. If your stumped, I’m in trouble. I will get on it.
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I’m assuming I have no grub installed. So, the sudo pacman -S install grub produced:
resolving dependencies...
looking for conflicting packages...
Packages (1) install-grub-2:2.12.r292.g73d1c959-1
Total Download Size: 0.01 MiB
Total Installed Size: 0.01 MiB
:: Proceed with installation? [Y/n] y
:: Retrieving packages...
install-grub-2:2... 6.4 KiB 35.4 KiB/s 00:00 [----------------------] 100%
(1/1) checking keys in keyring [----------------------] 100%
(1/1) checking package integrity [----------------------] 100%
(1/1) loading package files [----------------------] 100%
(1/1) checking for file conflicts [----------------------] 100%
(1/1) checking available disk space [----------------------] 100%
:: Processing package changes...
(1/1) installing install-grub [----------------------] 100%
:: Install your bootloader and generate configuration with:
# UEFI: https://wiki.archlinux.org/title/GRUB#Installation
# BIOS: https://wiki.archlinux.org/title/GRUB#Installation_2
:: Running post-transaction hooks...
(1/1) Arming ConditionNeedsUpdate...
cat: /tmp/install-grub.log: No such file or directory
rm: cannot remove '/tmp/install-grub.log': No such file or directory
Grub will be installed on: MBR
Installing for i386-pc platform.
Installation finished. No error reported.
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-5.10-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.10-x86_64.img
Found initrd fallback image: /boot/initramfs-5.10-x86_64-fallback.img
Found linux image: /boot/vmlinuz-5.4-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.4-x86_64.img
Found initrd fallback image: /boot/initramfs-5.4-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Windows Recovery Environment on /dev/sda1
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
Mod edit:- ``` before AND after the code.
Just passing through again.
I have to say you’re barking up the wrong tree with that logic.
Your machine can only boot as BIOS/MBR – it was made before UEFI was widely adopted.
You cannot “coax Manjaro Zetar (25) to boot in legacy mode” – Legacy booting is all your machine is capable of, and it’s the only option – you cannot boot UEFI/GPT.
I dare say a complete reinstall might be the fastest way to get back to a working system. In the meantime (to get windows booting again) you might consider booting with your Windows Installer and recreating your boot files, if the Installer allows that option.
Edit;- Except, now I recall that you’re using only one disk, this might not be ideal, so put a hold on that idea for a while.
No, you do have it installed, or else you wouldn’t have been able to boot.
But running the install-grub script is supposed to correctly install grub to wherever it needs to go — on a BIOS-booting system, this will be the Master Boot Record of the primary drive — and to properly configure everything.
Reinstalling Manjaro might be the option.
I’m not so sure about that. The install media are never fully up-to-date, so you’d still have to update your system again after reinstalling, and that will then reintroduce the very same problem.