Should be straightforward to a good visual basic programmer.
Something similar to [url removed, login to view] already have some coding done, but dont mind if you want to code from scratch. The tool would be in essence 2 things, 1 a desktop installer that runs in the in-tray, and has user settings to change announcement type to include sound, etc the other part of the tool (so we are talking about 2 softwares) is the part to send the messages, which would allow the [url removed, login to view] that pops up and the linked to message ... If you analyse the link above I think the tool will become clear. Subscribe and unsubscribe should be possible with the users tool.