So this is just windows magnifier with GUI and blacking out rest of the screen.
Turns out, windows magnifier does exactly just that and the app uses magnigier api. That's why it works in windows 8 and above only.
It is interesting to run 640x480 game on 27" monitor. it looks exactly like real 640x480 looked. The point is to run low res games in windowed and magnify with integers until they fit as much of the sceen as can be used without cutting anything. So with 640x480 I can magnify 2 times. So it still leaves me with black borders on all four sides(since 480p cannot fill 1080p screen up to any border) but the image is 2x the size of small 640x480 window and it's not smoothed out like normally in full screen.
Many emulators and source ports of old games already do that I think but 1920x1080 with filtering turned off is not exactl the same thing since it renders geometry at 1080p.
Looks at the Duke comparison - The megaton edition normally renders at 1920x1080p but if ran at 640x480 in fullscreen, image is soft. If you run it windowed and magnify it to similar level (either with the app or just windows magnifier... or in paint) this is the result.
For testing I just doubled the image in paint vs in photo viewer.
Look at health and ammo text and overall sharpness of the 2nd image.
http://screenshotcomparison.com/comparison/127079
edit: The comparison came out wonky but You get the idea 
edit2 sgort version - if You want to recreate that 26 dpi 15" crt look, now you can 
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