[ubuntu] User logged in?

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http://ubuntuforums.org – Hi, I have been using 64bit Ubuntu for about a year and in the past week twice my hard drive is active upon login longer than usual and so I go to reboot and a message appears saying unable to reboot (or shutdown) while users are logged in, authenticate to do so. I am running this Ubuntu system at home so I don't want anyone else logging into my computer especially remotely! Have I been hacked or is it a bug? Thanks for any advice. (Hardware)