I am trying to find a means of copying the contents of multiple files to a single file WITHOUT destroying or overwriting the existing contents of the destination file.

That is possible for plain text files only (suffixed .txt) otherwise you need third party utilities aimed to the specific archive (e.g. to append multiple pdfs).

In case of .txt files, use the TYPE command, i.e.

type source1 >> target
type source2 >> target

where the commands can be iterated by a FOR statement, e.g.

for %j in (*.txt) do type %j >> target

By the way this is NOT a DOS question as VISTA has no DOS at all, so better you post similar problems in Programming Forum.

If your original is a and the others are b and c::
copy /b a+b+c d
ren d a

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If at first you don't succeed, you're about average.

M2