well, mounting and everything else you can do from dosbox itself, the things you wrote in the "notepad file" in autoconfig are just automatically run in dosbox right after it's start.
anyways, when it starts at Z: drive it doesn't mean that your drive is not mounted, you need to switch to the C: drive in order to er.. switch to it. so just type c: to get onto the c drive
in any case, fallout is windows game, dosbox is dos emulator, you won't run fallout in dosbox..
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