Fallout 3 pc keeps crashing at last quest!!!?

fallout 3 was working fine for the whole game it was a little laggy but stiil worked fine but i am on the last quest : Take it back! but for some reason when i exit the citadel after maybe 20 seconds my game stops and crashes saying fallout 3 has stopped working. i have windows 7 and i have all the DLCs installed including broken steel. any help?