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Pk (avast! team) posted that they where effectively blocked from installing by MS. Its not just Avast! that is blocked either. Anyway, it looks like you just wont be able to install Avast! on Win8 Dev, you will have to wait till the Beta is released.
If there is a way to remove what is blocking Avast!, I have not seen any such method posted here.
Even if one was posted, it might have been removed, as whatever method you could use to accomplish this, it might well violate whatever TOS or what not you agreed to when you installed Win8.