Error during install
are you sure you're having the exact same error as everyone else in this thread? Perhaps your problem is a different one entirely.
You may have to compare the difference between windows 8 file structures to make sure you're getting rid of the appropriate "agent" folder/files.
In another thread, some people have claimed to have success by running their Diablo 3 or Starcraft 2 updates, which fixes the battle.net files to the most recent live versions so Hearthstone can use it.
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