QSettings has zero relevance to this and QService is for Symbian devices. In fact, your question has nothing to do with Qt.
What you need to do is place a *.desktop shortcut or link to the application in the user's startup folder. See:
http://standards.freedesktop.org/autostart-spec/autostart-spec-latest.html.
For example, if I want to start the application /opt/myapp/myapp_executable
at login, I would create a myapp.desktop
file with the following contents:
[Desktop Entry] Exec=/opt/myapp/myapp_executable
and put it in my ~/.config/autostart directory.
If you want it to be executed for every user at login, then you'd put it in /etc/xdg/autostart/. But again, check the XDG site because the directory can be different if an XDG environment variable is set that overrides the default.
Your question really belongs on http://superuser.com