Hi,

I have less than 3 hours of game time. Just arrived in Megaton, talked to everyone (mostly) then I try to go in the Water Processing Plant for the quest but once inside the game freezes and forces me to log out of Windows. I erased all my old saves from Steam just in case and verify/repair via GOG but it's still broken. It's annoying considering I bought the game for the second time because the Steam version's totally broken and everyone says with GOG it works just fine, no tweaking required. Maybe it's not a compatibility issue, I really don't know. I hope it's nothing.

So I had the same issue, and this solution from Bethesda worked for me:

Fallout 3 was not developed for use on PCs with more than two cores. If your PC has more than two cores, please also try the following steps:

Open your My Documents folder.
Under My Games, open the Fallout 3 folder and select the fallout.ini file.
In the fallout.ini file, find the following line: bUseThreadedAI=0.
Change this line to read: bUseThreadedAI=1.
Add another line after it that reads: iNumHWThreads=2.
Run the game.

Thanks, but I already fixed the issue and 2 others actually. I never thought someone would answer. I detailled what I did on another thread. Part of it was kinda my fault.