You may (actually) have more luck at Bethesda's Fallout 3 forums (there is a tech forum for specifically for people with PC issues). The PC version has some serious crash bugs and there are many threads there that approach different way to help alleviate the issue. Honestly, there is not just one thing to try here, and the list is not that short.
Supposedly, a patch is being released soon. How soon? no clue.
Anyways, that's my 0.02 on the matter.