Hi
Windows Subsystem for Linux on Windows 10 anniversary edition is new.
It will not install unless Avast is uninstalled (disabling Avast is not enough)
See: https://github.com/Microsoft/BashOnWindows/issues/800
Windows Subsystem for Linux is basically Ubuntu Linux on Win 10/64bit and allows Linux stuff to run native on Win 10; unlike Cygwin that requires a compile from source.
It appears that Avast does not understand this new Linux like environment.