I am playing it right now on my rig and went through the exact same thing. The problem is apparently with a patch they did, which has trouble with quad core CPU's.
To fix it, pay careful attention to the path here:
Open up the fallout.ini file in: My Documents\My Games\Fallout3
Find the line:
change it to:
Add another line after it and insert:
This will limit the game to 2 cores and prevent the engine bug from causing the game to freeze.
Download and install the latest klite codec pack from here: http://download.cnet.com/K-Lite-Me