...... eeeeuuuurhhhhrhhhh ... if you don't mount a C: you don't have a C: in dosbox, and if you don't have a virtual C: the install prog can't write on your C: ., ahem.
As you're on Vista take a look there:
http://www.abandonia.com/vbullet/showthread.php?t=19982
Will avoid you lots of problems, certainly if you begin to install games from flops.
Most flops use batch files or call them to create-install-write folders-files.
Best see that dosbox has the needed permissions under Vista.
So, basically:
* You MUST always mount (or automount in [autoexec]) a virtual C:
* Your
mount a a: is not 100%
Should be:
mount a a: -t floppy
Give it another try now, with a bit of luck it will work.
However some ol'batch installers use commands that are not implemented in dosbox, so it's trial and error here ......