revert dual boot

[claydoh]

Normally there is a sticker under or at the backside of the computer with the licence code.

Not always, my last two bought-new laptops and HP consumer PC did not have this, and my two "business" calls mini HP PCs definitely do not (but these would be using a volume license I assume, from the company who originally purchased them.)

It appears that I have an EFI partition on the Windows disk, but not on the Linux disk. I am not sure what the implications of this are

Not a whole lot, really. If you were to unplug the Windows drive, Kubuntu or any OS on the other driver won't boot. Bios boot order preferences would still be managed the same way, and the bios boot selection menus/shortcuts would work the same.

Also, it is not at all clear to me how I would find the microsoft license

https://support.microsoft.com/en-us/...1-a806abf0a886
But more likely:
https://www.techrepublic.com/article...0-product-key/
https://www.tomshardware.com/how-to/...ws-product-key
OEM licenses may or may not have issues activating in a VM, though it may not happen immediately. The license is tied to the hardware.
An unactivated Windows will have a watermark on the desktop, and you won't be able to change the wallpaper, and a few other settings will be missing. But for your purpose of running Turbo Tax, this should not be affected at all.
I am confused about what happens if one digs out the key from a physical computer and uses it it on a different system (a VM), and what happens to the original physical instance/activation (the physical computer), if that install is kept around and used
https://www.tomshardware.com/how-to/...ws-product-key

However, licenses can be had legitimately for quite cheap. I think my digital license cost me 14 dollars or so, and I have used it for quite a while now, even upgraded to Win 11 at no cost. I cannot recall where I got it exactly. I have used other sources in recent years when selling hardware, and have had no complaints. Vendafly and Digital Chill Mart (bad names, for sure).
It is difficult determining which vendors are legit; I wish I could find where I purchased my personal one.
Not recommending spending $$ here, but it is an option that may suit you.

As to using VirtualBox or other software for virtual machines, there are gobs of how-tos out there, so it would be easy to install Windows there, and see what happens or how things work if/when it decides to become in-activated.
Virtualbox is readily available from the Ubuntu repos, though it is an older version in 22.04. While that should be fine, installing the current version will probably be a slightly better experience
https://linuxiac.com/how-to-install-...-ubuntu-22-04/
https://itsfoss.com/install-windows-...tualbox-linux/

[monon]

Thanks to all of you who have helped me on this issue. I was able to go googling and download a Windows 10 ISO image. Seems like I am ready to rock. I appreciate all the help.