I have Avast running as my anti-virus and have had no issue installing or running 2.5.1.15 (or versions before this for the last 5 months or so I have been using Avast). This is on a W7 Home laptop. Neither do I recall setting anything specific in Avast to allow SGP installs.
Sorry if this throws a spanner in the works 
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Any logging I can do to help?