There is a mod on nexus that will make your windowed mode full screen
http://www.fallout3nexus.com/downloads/file.php?id=45
Thanks for that. Unfortunately though windowed mode seems to be playing up now. Though have found another fix that seemed to work (though not tested for long periods). People are saying that after patch 1.1 the game doesn't like 4 cores anymore, & to get around this you have to edit the fallout.ini file to limit the game to use 2 cores,
Open up the fallout.ini file in: My Documents\My Games\Fallout3 (make backup on fallout.ini file - just in case)
Find the line:
bUseThreadedAI=0
change it to:
bUseThreadedAI=1
Add another line after it and insert:
iNumHWThreads=2
The above seems to work (for now), and when I switch back to the old fallout.ini file it starts to freeze again (hopefully indicating that was the problem).
I've also found information regarding a windows vista hotfix that fixes a known issue regarding certain system and application crashes on computers that have multi-core processors. I haven't tried this yet though so don't know if it works. Below are the links to the fix. Top is 32bit, bottom 64bit.
http://thehotfixshare.net/board/index.php?showtopic=11094
http://thehotfixshare.net/board/index.php?showtopic=11095