[texhax] TeX or LaTeX "engine" for Windows PC

[Karl Berry]

    interested in using an existing TeX package such as MiKTeX

If you install a minimal subset of miktex (or tex live), you'll pretty
much have "bare bones" TeX.  A binary by itself is of no use; it needs
many supporting font and macro files to actually typeset anything.
That's what the distributions are all about.

The source code is included in all the distributions.  Knuth's original
program is a "literate program", pascal + tex.  Most of the additional
code is written in C.  Hooking it all together is not trivial.

Also, as it happens, this same question (about typesetting math from an
outside program) was asked and answered recently on texhax.  Check the
archives.
http://tug.org/pipermail/texhax/

http://tug.org/begin.html has beginning points for a number of aspects
of TeX.  Browsing around the TeX faq at http://www.tex.ac.uk/faq may
also be helpful.

Best,
Karl