Ubuntu 12.04 shut down freezes

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http://askubuntu.com – I am using Ubuntu 12.04 for last one month without any problem. It was upgraded from Ubuntu 10.10. Recently I needed to replace my hard disk. After replacement I installed Windows 7 and Ubuntu 12.04 on the system. For some days it went ok, but now shutting down from Ubuntu is causing problems. It freezes on the text screen saying acpid=exiting checking for running unattended upgrades. After that I waited for a long time, but it didn't go off and I had to hit ctrl+alt+del to restart the system and booting from Windows and then shutting down from there. This happens every time I shut down my (HowTos)