[ubuntu] Can't shut down my computer

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http://ubuntuforums.org – My son in law called round today whilst I was at work and used my computer. Dunno what he has done , but now when I select shutdown, the computer doesn't shutdown, but goes to the login screen. I've checked the settings and am convinced it should shutdown completely when I select turn computer off. Any Ideas ?? Only thing I can do at the moment is force a hard shutdown using the power button on my computer (which I'm loathe to keep doing) (Hardware)