I need to find some free (preferably open source) forum/bulletin board
software available on the internet for download that supports dynamic
cross posting.

For example, if there are forums A, B, C, ... Y, Z, there has to be a
way to make a new message that gets posted to certain ones, say, B, G,
M and Y.   This can?t be done by making a group that includes B G M
and Y ahead of time, it has to be done dynamically, at the time of the
post.  A message will be written once, and somehow it?ll be posted on
several dynamically selected  pre-existing forums.

Or if there is a way to do this with 'announcements', that is fine
too, writing an announcement once and sending it to selected forums;
but again, the forums the announcement is sent to have to be chosen
dynamically.

Other requirements:  
-This should run on Windows IIS, and could be backed by MySql, Access,
or most anything else I suppose, as long as there are no
obscure/costly pre-requisites for installation.
-Username/passwords for login, so users can only have access to certain forums.
-The more moderator-configurable options the better.