This worked for me on the newest version of windows 10 so it wil probably work for you too.
Fixing the “fallout3.exe” file
First you want to go to the game directory of fallout 3 open the properties and turn on compatibility mode for Windows XP (Service Pack 3), i havent tried Windows 7 but i found this worked so i didnt feel the need to.
Next you to set the .exe to Run with administrator and Disable full screen optimisations if you don’t plan on running the game in fullscreen you don’t need to disable full screen optimisations but if might as well anyway.
Note: Even by disabling full screen optimisations the game might still crash when you quicksave, its doesn’t freeze it crashes which means the when you load the game back up you will have lost no progress. This can be fixed by using Nexus Mod Manager to install the mod “CASM” which requires “FOSE” which i will not go into in this guide but this isn’t that big of an issue.
The left is the properties window and the right is a game directory.
Fixing the “FALLOUT.INI” file
Once you have fixed the .exe the game will load however it will crash a lot due to the game only using 1 Core of your CPU.
You want to find the “FALLOUT.INI” (Not allways in uppercase weirdly) edit with notepad++ if you don’t have notepad++ its not hard to install and i would recommended getting it as it gives line numbers, you can get it here.
https://notepad-plus-plus.org/download/v7.5.8.html
On line 88 the line will read “bUseThreadedAI=0” you must set the to “bUseThreadedAI=1” then make a new line under it and type “iNumHWThreads=2” that will enable 2 cores, if this still crashes set it to “iNumHWThreads=1” will fix it (Which i didn’t need to do myself)
Right is the notepad++ after the changes are made and left is the file dir without FOSE installed.