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[The Fifth Horseman]

To circumvent this problem, you have to mount a CD-ROM drive in DOSBox.

It doesn't really matter whether that's your physical CD-ROM drive, a directory mounted with the -t cdrom argument or an ISO image mounted through IMGMOUNT - any of these will make DOSBox load it's CD-ROM drivers and thus allow the game to run.

mount d [YOUR CD DRIVE LETTER]:\ -t cdrom
Is the most practical, since it also lets you use your real CD-ROM drive in DOSBox. Or you can also use the CDEMU utility that is included with some of the downloadable versions of the game (including the one here, IIRC):
cdemu2 D: