I took your code, put in qt creator, build and run it and the tray shows on the system tray but without an icon which is understandable because i dont have that icon in that path but the tray does not respond to clicks but atleast now i have an output and it says:

Object::connect: No such slot MainWindow::trayClicked(QSystemTrayIcon::Activatio nReason)

so i guess this is progress.

What tool did you use to write, build and run the code in? This could be a Qt creator 2.3.1 + Qt 4.7.4 specific issue