Raising the MainWindow to active/focused state

I have an issue where I have two processes on the same system, and one of them, on encountering a particular situation, needs to tell the other to make itself the active window. The first process is written in C, and the second, my process, is written in Qt 4.5.3 under VS2008 on Vista. I have looked for a way that, when I receive a signal from my partner process, I can cause the MainWindow to raise into active/focused state on the system display. I see how any widget that has a Qt parent can be made to do this, but I see no way to tell MainWindow to raise itself into this state. Can anyone help me? This is a mission critical process relationship, and if the Qt window cannot be made to become active in very specific situations, then the overall application fails to work as it should.
Any help is highly appreciated. This response eventually needs to be made to work on Linux and MacOS systems as well.
Bruce