Sp4zzmatica: I'm naturally cautious about these kinds of programs, and I trust my anti-virus software. Can you assure me that the program is safe?

volcanotron: Yes lad it's fine, the guy in this thread used it, I use it and I submitted this solution to gog to double check if it works or not and it did which is why it's pinned here. If you run into any problems let me know and I'll see if can fix it

volcanotron: https://www.youtube.com/watch?v=MqF63EsJRlQ video of the process from a fresh install

Fullscreen mode:
download dgVoodoo
extract somewhere
launch dgVoodoo and press the Add button on the right of config folder and point it to the R6 folder
General tab: appearance fullscreen,scaling mode unspecified,capture mouse. pretty sure its just default for this screen
DirectX tab: videocard dgVoodoo 3D, vram 1024, if you want turn filtering to max and msaa to 8x.
Make sure you set resolution to unforced otherwise the game will blur when you move the mouse around in game
press Apply
go to dgVoodoo folder and open MS folder and copy the 3 files D3D8,D3Dlmm,DDraw.dll and put them in the folder where rainbowsix.exe is

go to the registry and find r6 mine was located under
HKEY_USERS Software\Classes\VirtualStore\Machine\SOFTWARE\Wow6432Node\Red Storm Entertainment\Tom Clancy's Rainbow Six

Fullscreen TRUE
VideoResolution 1920x1080

while you're here change MouseRadiansPerPixel to 0.002727 or whatever setting is fine for you by default i think its 0.008727 which makes your crosshair really unstable and the sensitivity through the roof

I would also suggest capping the games framerate to 60 by whatever means. I use nvidia Inspector for mine r6 sieges profile works for the original game for some reason. if you don't the AI both enemy and friendly will often run in circles
2-Sync and refresh
frame rate limiter 61.8

Windowed mode:
follow the above up to the registry step but change
Fullscreen to FALSE
VideoResolution 1600x900
do not use your max resolution for this if it goes out of bounds it will crash, you can make incremental changes if you want to make it as close as possible.

the menu part of the game will still be small but once you load into a map it will change to the desired resolution

the only compatibility setting enabled is 16 bit colour

I really hope this works for you, if not tell me what the error says in the Sherman.log file located in the folder with rainbowsix.exe