Cannot open anything (like Steam) or shutdown/restart

Hi, I'm using a Windows 8 computer, and sometimes this occurs randomly (kind of frequently). My computer basically "freezes" and I can't open anything at all. (ex. Steam, Battle.net launcher) However, I can open my browser and use it unaffected at all. When I manage to open one of the apps like Control Panel, I click one of the button and it freezes up, not to mention I can't open task manager either. Then when I try to shut down my computer or restart it, it gets stuck in the "shutting down" or "restarting" forever. I never tested how long that lasted, I usually just end up holding the power button and shutting it down manually. I have Norton 360 antivirus and i scanned my computer, but it didn't find any viruses. So I don't know what is wrong, because this happens randomly at times. Please help :l

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