You can fix the game by applying a compatibility tweak. Right click the executable file and select Properties > Compatibility. Use Windows XP Service Pack 3 and accept the changes then (if you have a quad core computer), locate the fallout.ini file in My Documents\My Games\Fallout3.
Identify the following options :
bUseThreadedAI=0 > change to 1
iNumHWThreads=2 > change to 2 from 4. If you still have problems, change to 1
Close the file then start the game again and it will work.