I have made it to the Citadel, and found out about vault 87. I have trained in power armor, and been granted trading rights. Turned in holo tags etc. I also have applied the .ini fix to the Fallout.ini file in my documents for possible threading crashes. I have uninstalled and reinstalled my copy of Fallout 3. That being said... I can run around inside the citadel all day long, but if I try to leave, I get the loading screen, and then crash to desktop. Windows searches for a solution and cannot find one. I am running windows 7 on the bootcamp drive of my macbookpro. up until this issue i have had no problems playing after I applied the .ini fix for threading.

Running any mods? Sounds like its trying to load something out there that it can't find.

Try a new game or earlier save.

running no mods, and stupid me did not save before going into the citadel... I tried a few saves from inside, before training power armor or gaining the knowledge of vault 87, and got the same result....

I cheated, I just fast traveled from the courtyard to Megaton. problem solved.