I cannot imagine how DOSBOX could do this (I do not use D-Fend but I also doubt that it would do that).
Remembering past occurences when DOSBOX was falsely blamed for seriously screwing up something, I think that with 99 % probability the problem has nothing to do with DOSBOX.
Mounting in DOSBOX is done internally in DOSBOX, it has no influence at all on outside system. DOSBOX also does not install any system libraries etc.
No one else reported this type of problem (and there are thousands of people using DOSBOX).

Mirek

Make sure that autoexec.nt contains the following (assuming Windows XP):

@echo off

REM AUTOEXEC.BAT is not used to initialize the MS-DOS environment.
REM AUTOEXEC.NT is used to initialize the MS-DOS environment unless a
REM different startup file is specified in an application's PIF.

REM Install CD ROM extensions
lh %SystemRoot%\system32\mscdexnt.exe

REM Install network redirector (load before dosx.exe)
lh %SystemRoot%\system32\redir

REM Install DPMI support
lh %SystemRoot%\system32\dosx

REM The following line enables Sound Blaster 2.0 support on NTVDM.
REM The command for setting the BLASTER environment is as follows:
REM SET BLASTER=A220 I5 D1 P330
REM where:
REM A specifies the sound blaster's base I/O port
REM I specifies the interrupt request line
REM D specifies the 8-bit DMA channel
REM P specifies the MPU-401 base I/O port
REM T specifies the type of sound blaster card
REM 1 - Sound Blaster 1.5
REM 2 - Sound Blaster Pro I
REM 3 - Sound Blaster 2.0
REM 4 - Sound Blaster Pro II
REM 6 - SOund Blaster 16/AWE 32/32/64
REM
REM The default value is A220 I5 D1 T3 and P330. If any of the switches is
REM left unspecified, the default value will be used. (NOTE, since all the
REM ports are virtualized, the information provided here does not have to
REM match the real hardware setting.) NTVDM supports Sound Blaster 2.0 only.

REM The T switch must be set to 3, if specified.
SET BLASTER=A220 I5 D1 P330 T3