On Random Mutations

As I understand it, random mutations are
currently held to be responsible for most
genetic changes, and random mutations are
overwhelmingly caused either "spontaneously"
or induced either by chemical agents already
present within that particular organism or from
the outside by cosmic radiation colliding with
genetic material, triggering off chemical
changes and thus effecting a purely random
change. We are also asked to believe in
Darwinian evolution.

Excuse me - but it does not make
any sense to me!

Think of the following experiment:

We have a computer programme, of average
size and average complexity, fully tested and
proven to "work as designed".
We now print out the actual code that is running
when the programme is being executed, be it in
a high-level user-friendly language or something
closer to the machine, like assembler.
We then hang this printout on the wall, either
vertically, which could prove difficult because of
the contigious paper, so hang it sideways around
the room, whichever way suits.
Then take a few steps back and either fire a
shotgun at it or just a single-shot pistol but keep
shooting until you see at least one single
character has been damaged or removed.
Now take the paper down, scan it in carefully
sheet by sheet and attempt to re-compile or
re-assemble it, as this is a prerequisite to
executing the new version.
As a former software consultant over many years
I guarantee you that our programme will not even
compile/assemble which of course means that it
will never run again.

What is the likelihood of any purely random
change actually improving our programme?
Surely non-existant/zero/nil.
If this could be made to work at all, surely any
random change would be harmful, possibly
possibly -on a lucky day- neutral in effect.

This is of course a very crude comparison with
the terribly complex fabric of life itself but doesn't
that make it even less likely that purely random
changes have any possibility at all of effecting
positive changes which might lead to advances
for that particular living organism? I think so.

It seems pretty obvious to me that any random
change would be harmful and therefore lead to
malfunctioning and/or disease and from there
fairly quickly to extinction!
If the above conclusion is correct, it follows that
the whole concept of random mutations being
the main agent responsible for positive
evolutionary changes is absolute rubbish!!