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 system from Ubuntu. I have seen many posts regarding the same kind of issues but none of the solutions reported there helped me out.