Hmmm…

  1. Remove NAV through Add/Remove programs from Control Panel. Boot.
  2. Use Norton Removal Tool for Windows 2000/XP/Vista.
  3. Boot.
  4. Install avast! Boot.
  5. See what you get.

You don’t have to… It will ask you automatically when (if) needed. But in this case of WebShield, I don’t think it’s needed.