Even though making an executable using a compiler like VB
or C or Delphi will make it more difficult to track the
mathematical formula, there exist decompilation programs
which probably make it relatively easy to find out how
things are computed.

I think that an important point that you should take
account of, is the fact that many people around the world
use Excel (whether we like that or not), and that it
greatly improves the ease-of-use if you distribute Excel-
compatible applications. Maybe you can combine the two by
trying to hide your magic formula in a DLL or something
which can then be used in combination with Excel. In this
newsgroup however, you can see that problems with home-
made DLL's are quite common, but I have never tried it
myself. Maybe expert advice