Fallout 3 is a great game but it can suffer very badly from crashes if you installed any codec packs. I recently bought the DLC pack “Operation Anchorage” and the game crashed all the time on me and was unplayable. This happened in the Orignal game as well but it wasnt as bad. Anyway after some searching i found this tip on a forum that has solved the problem for me;
Fallout 3 does seem to have a conflict with FFDShow codecs. You have to
either uninstall them from your system or add fallout3.exe to FFDShow
audio and video decoders exceptions list. Although you may not know it,
FFDShow codecs may be present on your system if you have installed
codec packs such as CCCP (Combined Community Codec Pack).How to add Fallout 3 to FFDShow decoder exceptions list
1. Launch the ffdshow video decoder configuration window. You may
access this through the start menu. If you have installed any codec
packs, check the start menu entries of those. If you can�t find it on
the start menu, right click the ffdshow video decoder (red) icon that
appears on the taskbar when a supported video file is playing, and
select �ffdshow video decoder� option from the menu. ffdshow_icon2. Tick the �Dont use ffdshow in� checkbox if not already checked
3. Press the edit button that is next to the text field. A new dialog
box will pop up. Press the Add button. The �Add incompatible executable
file� dialog box will be displayed. Browse to the Fallout 3
installation directory and locate fallout3.exe file. Select it and
press open. Close all dialog boxes currently open by pressing ok.(OR you can simply type fallout3.exe at the end of the text field next to �Dont use ffdshow in� checkbox)
4. Repeat steps 2-3 in ffdshow audio decoder configuration (the blue
icon in taskbar) window as well. It is important that you add
fallout3.exe to both ffdshow audio and video decoder�s expections list.