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  1. Please, browse the Symantec website and download the specific uninstall tool for NAV. It will clean trash left behing by NAV (files, registry keys). You need this to proper working with avast.

  2. On ‘Control’ in my signature you can download AEC and there is an option for Norton uninstall too.

  3. Can you try to repair your installation?
    Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
    Then choose Repair function in the popup window (Repair).

If this does not help, can you uninstall / boot / install / boot again?