If you want the TL;DR: Launch the program, put in your Fallout 3 directory, pick an option, and press the Patch button. 

A UI that will help you either disable Games for Windows Live, or fix it for achievement support.

There are many things it will do, and 2 things it can do.

As of Version 1.1, the CMD window will no longer show up, as it now loads the required files itself.

UI:
As you can see, it looks like a Windows Forms app. It is.

Until you insert a directory that contains the Fallout3.exe executable, the options are grayed out.
Thankfully, there's a way to open a dialog, and it's not the one where you have to go through a tree of folders and makes you want to vomit.


Option 1: Disable GFWL
The program will (in order):
* Copy Fallout_default.dll to Fallout.dll in your Documents folder it's not there already
* Open the GFWL Disabler program from https://www.nexusmods.com/fallout3/mods/1086

Option 2: Fix GFWL
The program will (in order):
* Put the GFWL Client and Xlive Redistributable from https://www.nexusmods.com/fallout3/mods/24699 into the game's "temporary" folder (btw, it just made one)
* Put the Registry tweak from https://www.nexusmods.com/fallout3/mods/24703 into the game's "temporary" folder
* It will install the GFWL Client and Xlive Redistributable
* It will apply the Registry tweak
* If the graphics options haven't been set, it will open the Launcher to do so
* It will change the graphics options to have your actual screen width and height (I did this for my own sake, I was using a VM for testing, but it will probably help someone)
* It will put the product key the game will ask you in your clipboard, and tell you to press Ctrl+V when it asks you
* It will open the game, and ask the user to go to the screen where you put in a product key
* Once entered, it will update LIVE
* It will open the game again
* It will ask the user to sign in with Microsoft again, and then exit itself