This windows demo app from 2004 shows you how to use the winapi EnumWindows function in association with SendMessage to send messages to a window.

The application works with Mirc (an IRC client) and Trillian (a messenger program that works with msn,yahoo,icq and aim). It hasn't been tested with the latest versions of Mirc/Trillian but it still demonstrates the message sending.

Get mirc here
Get trillian here