I no longer get the system-settings option on my ubuntu 12.04. I have earlier used it several times without any problem. I am not aware which action could have caused this problem but I don't remember uninstalling or deleting any package recently.
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sudo apt-get install ubuntu-desktop
The above command will install everything, which comes as default settings(software center,system settings)
sudo apt-get install gnome-control-center
This will install system-settings option