Ok, you’ve done the right thing 
No, I want to know the local path you’re installing avast… something like:
C:\Program Files\Alwil Software\Avast4\
Please, follow:
- Remove NAV through Add/Remove programs from Control Panel. Boot.
- Use Symantec removal tool following the three steps defined in the SymNRT tool info or here.
- Boot.
- Install avast! Boot.
- See what you get.