If you're running Fallout 3 on a dual- or multi-core solution and experiencing frequent lockups or crashes to the desktop, make the following changes:
Open the folder 'My Documents' / 'My Games' / 'Fallout3'
Look for and open up the file FALLOUT.ini.
Inside FALLOUT.ini look for a group called [general].
Inside the general group, after all the configuration parameters, just copy and paste the following two lines:
Now save your work, close FALLOUT.ini, fire F3 up and get ready to enjoy the damn game the way it was meant to be played.
Props to TweakGuides for their outstanding and exhaustive workup, and thanks to Bethesda for bringing Fallout into the 21st century with style.