I'm posting this solution here to help other goggers with my same problem.

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When playing old 4:3 games fullscreen, I often prefer black bars, more than an ugly stretched picture.
To obtain that, I usually have to simply change my graphic settings.

However, I couldn't manage to obtain this from the Nvidia panel in "Adjust desktop size and position".
The option "Aspect Ratio" kept getting back to "Full-screen" for no reason.

Well, I finally found this old post about it:
https://forums.geforce.com/default/topic/882212/geforce-drivers/howto-set-resolution-scaling-to-maintain-aspect-ratio-on-new-buggy-nvidia-control-panel/

So, it's an old Nvidia bug, not yet fixed after years!

I'll copy the text here, in case the link breaks in the future:
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This topic explains how to make 640*480 and similar resolutions not stretch horizontally on a wide screen. Buggy NVIDIA Control Panel of the latest drivers resets scaling to "Full-screen" whenever you set it to "Aspect ratio", at least for me. Below is a fix that requires using RegEdit, so you may want to create a restore point before doing it to feel safer.
- Set "Perform scaling on" to "GPU" in NVIDIA Control Panel.
- Open RegEdit with admin rights (use search in Start menu, then Ctrl+Shift+press on it).
- Navigate to "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration\(long string of symbols that describes your device, normally the only folder out there)\00\00".
- Change "Scaling" to "4".
Done! No restart required.
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P.s: with multiple monitors, you'll have to identify the correct one and then edit all the combos.
Suggestion: export to .reg, change "Scaling"=dword:00000004 via notepad, and then import back.

P.p.s: the fix resets if you change the options again from the Nvidia panel. Keep the .reg to easily restore it.

I make a custom dosbox profile in Nvidia Control panel and that usually fixes it for me I put it under the scaling options scale windows so the edges fit the screen seems to fix it for me and I fiddle and tweak until the games are screaming along!

for quake I either use 1152x864 or 848x480 and set the dosbox as speed as 60,000 makes it buttery

wish GOG would get with it though and use source ports for these games like

http://nquake.com for multiplayer

and

or even this: http://fte.triptohell.info/moodles/win64/afterquake.zip

and for single player there is a little known mod called ARCANE DIMENSIONS

and to play this SEXEH game mod you'll need QuakeSpasm v0.93.1 win64 version

or

QuakeSpasm v0.93.1 win32 version

For Linux:

Linux AMD64 Version

or

Non AMD64 Version

Finally for MAC

OsX-SDL2 Version

or

OsX Non SDL2 Version

This post needs PINNING!

Here is a gameplay Video Here https://www.youtube.com/watch?v=mcS0v5286t4

Try to post this into Geforce forums or contact nVidia support

i suggest to use latest version of DDU (read use instructions)

for ms-dos version of Quake1 remember to use v-sync or to cap frame rate to 60 or 72 fps, not more (windows version works without speed problem)

DOSBox support and use only 60hz of display refresh if i remember; ms-dos use 70hz of display refresh

It's already posted there, even if noone replied to that guy :\
It took me a lot to find that lonely thread.

Do you have the same issue I explained here?
If not, it could be a bug not affecting your system.

I wonder why it affects some people and not others? I've been using Nvidia cards and widescreen monitors since 2006 and I've never encountered this bug.

Dunno, It seems a normal possibility to me.
Bugs don't always affect all systems at once.
Some evil ones could even be specific to some few configs.
That's probably the reason why Nvidia didn't care to fix this issue in 3 years.. -_-

Btw, in the meantime, I found this other problem related to Nvidia:
https://forums.geforce.com/default/topic/844905/geforce-drivers/-feature-request-nonblurry-upscaling-at-integer-ratios/

Blurry integer upscaling.. I wonder if at least this has been fixed now. -_-'

Useful. Thanks! Added to the favourites.

AMD is no different...
I don't know about Intel, but I suspect it's the same.