Earth is round (or so Copernicus will have us believe) and that sucks.  Why?  Because we can't use c=Square root of (a² + b²) to figure the distance between two points.  So in order to perform this calculation we must use 3D trigonometry.  Linked below is a project that does just that.  It comes with an external C DLL.  The reason the DLL was written in C is because the calculations proved to be too slow in VB.  The DLL is freely distributable.

Download Distance.zip