The presence of NAV. It’s impossible to live with avast (or any other antivirus).
Disable is not enough. It never is. As David said.

  1. Remove NAV through Add/Remove programs from Control Panel. Boot.
  2. Use Symantec removal tool following the three steps defined in the SymNRT tool info or here.
  3. Boot.
  4. Install avast! Boot.
  5. See what you get.