I have a new Asus laptop and couldn't get fallout 3 to run because it tries to run the core i5 as a quad core. Here's a fix I found on another website, it worked like a charm!

Problem is that the i5's have 2 cores + 2 HyperThreading cores. So as far as windows see things you've got 4 cores. Fallout seems to have problems with quad core systems. Here's the fix:

Open up the fallout.ini file in: My Documents\My Games\Fallout3

( backup the file as a precaution....)

Find the line:

bUseThreadedAI=0

change it to:

bUseThreadedAI=1

Add another line after it and insert:

iNumHWThreads=2

Hope that's some help!