williebzaerka is right. In order to play this game you have to modify one line in the FALLOUT.ini file. This file should be located in C:\Users\COMPUTERNAME\Documents\My Games\Fallout3.
Find the line bUseThreadedAI=0 and change it to bUseThreadedAI=1
You will also need to add in an additional line of code after this one called: iNumHWThreads=2
This will limit the game to 2 cores and SHOULD prevent freezing/crashing.
If this doesn't work let me know and I'll do some more research.
Sorry for taking so long but I've found another probable solution. You need to run the game as an administrator (right click) and some sound cards can cause crashing so disable those. Also make sure Fallout 3 is fully updated and then change the code lines I listed above.