Maybe a stab in the dark , but try deleteing you Fallout.ini and FalloutPrefs.ini found in "Mydocuments\Mygames\fallout3"
Look for another .ini found in saves also.
And then run the game through the intro. Ofcourse if you had edited stuff within those files , youll have to redo them.
But if that doesnt work , im guessing an uninstall reinstall will fix it. All mods and DLC's will stay put though.