Now I am not sure if many people do or do not know this, but fallout 3 tends to ctd for varying reasons, but the one I found that was pestering me was, that I had more than one monitor on, and I remembered that oblivion used to do the same thing back a while ago, and the fix for me was just turning off my other monitors, and keeping my main one turned on, this fix apparently also applies to fallout 3, go in to control panel, and turn off the other monitors, and only use one for fallout 3, and see if that fixes your start up ctd, hope this helped.