A user has multiple applications running on his/her PC and I want to create
a broker component or service that is shared among the multiple apps.  Each
time an application changes a record it notifies the broker and the broker
in turn notifies the other applications so they can take appropriate
actions.  Any one app may send a string data to the broker and the broker
makes this string data available to all the other apps.  This has to happen
instantaneously.

1) I would prefer to use .NET C# to develop the broker component .
2) The broker component must be able to raise event (to notify) in any of
the multiple applications (or use callback functions?)
3) The broker component also run on the user's PC and only one instance of
it is shared by all applications.
4) One of the applications was written in Java.

My question is what should this broker component be created as (project
type)?  A Windows service, a Serviced Component, Message Queue Component or
just Class Library?  Keeping in mind that some of the apps may not be able
to work with the Message Queue component for example.

Thanks
Tom