When it comes to screen stretching there's an easy way to fix that:
Your monitor probably has a status menu that shows what resolution is fed into it. If you know how to, get that up and see if it differs when you are in Windows versus a game. If it differs, it's your monitor that stretches it. If not, it's your graphics processor that's doing it.
If the resolution does differ, look in your monitor's onscreen menu and see if there is a setting like "aspect ratio" , "panel fill" , or something to the likenesses of that. Because wording depends on the particular monitor, just fiddle with the options until the image looks right. A 4:3 signal should have black bars on either side, and a 16:9 signal should obviously fill the whole screen.
If the resolutions don't differ, check your gpu settings. Since there are so many different configuration programs, it would be best to do a Google search on how to change that. If you don't know what gpu you have, open the device manager and look under the "display adapters" node. You should see the type of gpu your computer has. Put that, along with "aspect ratio", in a Google search, and you should find out how to set it properly.
However, while "Enable GPU scaling" is toggleable, the options for [maintain aspect ratio / scale image to screen / use centered timings] are grayed out.
It will do that if the resolution is set to the maximum size of your monitor. Switch the resolution to the lowest it gets and you'll see that the settings aren't grayed out anymore. Now select "maintain aspect ratio" and set the resolution back to the maximum.