There are two versions of Fallout -- one for DOS and one for Windows. IIRC, most releases had them on the same disk (checked my copy, it has both). Maybe you tried to run wrong executable under DOSBox. You have to run installer under DOSBox as well, Windows installer may skip DOS section completely.
Are you talking about Betrayal at Krondor or Return to Krondor? If it is the former, it should run perfectly under DOSBox if installed properly; if it is the latter, unless you really must to play that one, I'd advise playing something else... In any case, it's Windows only, and it had compatibility issues even when released. Trivia: CGW review of Return to Krondor was named "Return to vendor."