Microsoft Internet Explorer has been freely available since 1995.  In fact, it has been pre-installed on every new personal computer using any of the Windows operating systems since 1998.  It coexists with other browsers and does not need to be used as a browser per se.  The requirement for its installation is simply to allow for the proper installation of the DLL's and custom controls used by for the HTML Help utility used by The Compleat Botanica.

Microsoft SQL Server Desktop Engine is also freely distributed.  It is simply the best embedded database for optimized queries and very large databases running on the Windows platform.