I set the password for avast.
In principle, I need to type the correct password to stop avast working or change the setting.
But I discovered I just have to type thousands of words when avast ask for password and it will stop its service.
And then, I can change anything I want.
It just like password doesn't exist.
PLEASE FIX THE PROBLEM ASAP.
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This video shows everything
http://www.youtube.com/watch?v=y3Fl5w2nLPg