FFDShow
FFDShow is a popular free software package containing "codecs" -- short for "coder-decoder" -- that enable your operating system to play a variety of video and audio files. In some cases, FFDShow is not compatible with the PC version of Fallout 3, and FFDShow users may experience computer crashes while playing the game.
Opening the FFDShow Configuration Program
If you have FFDShow installed, its configuration program should be accessible from the Start menu. Click "Start," type "ffdshow," and look in the search results for the two FFDShow configuration programs -- one for audio and one for video. Open both of them and make changes to each.
Adding Fallout 3 to the Exceptions List
In both the Video and Audio configuration programs, check the "Don't use FFDShow in." This appears next to a text field containing a list of executable files -- when the box is checked, FFDShow will be disabled for all of the programs in the list. Click "Edit" next to the list, then navigate to and select the Fallout 3 executable, which is usually located at C:\Program Files\Bethesda Softworks\Fallout 3\fallout3.exe. If you specified a different location when installing Fallout 3, look there instead.
Removing FFDShow
If you have followed the instructions above, but still experience crashes while playing Fallout, try removing the FFDShow codecs entirely. This will likely cause your system to be unable to play certain video and audio files requiring the FFDShow codecs, but since FFDShow is free, you can always redownload and reinstall it. Click "Start," then "Control Panel," then "Programs," and follow the instructions to remove a program, selecting "FFDShow" from the list when it appears.