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First, you could try using the appropriate removal utility for the specific AV you used to had (even if was a long time ago). Check for some removal tools at http://thewebatom.net/uninstallers/security-software/, download it, then reboot into Windows Safe Mode and run it under that condition.
Once again in Normal Mode, got to Avast main GUI → Web shield (activate it) → EXPERT settings → “…well known browsers only”.
Please reboot, test and report back.