Overview
This guide will help you disable the annoying mouse acceleration, so you can enjoy Fallout 3 without it.
Introduction
Now someone else may have already made a guide on this, but I wanted to post it here just in case.
So recently, I started playing Fallout 3 again, and the mouse acceleration really got to me, so I found a way to fix it.
Fix
Go to your Fallout 3 install directory you can get here by going to steam library right clicking on Fallout 3 then properties, local files, then Browse, find Fallout_default.ini. Open it using notepad, and scroll down to the section titled [Controls].
There, paste the following:
fForegroundMouseAccelBase=0
fForegroundMouseAccelTop=0
fForegroundMouseBase=0
fForegroundMouseMult=0
Then, just open the game and enjoy.