The fact that I blame into Norton comes from the previous installation of Norton SystemWorks into my system. I have all utilities except NAV. The Norton key have two groups os zeros and avast could not add the driver there. I just change the lines, avast before and Norton after.
Again, if you change config.nt files you won’t be protected.
You can have problems to run 16bits applications (old pkzip for instance).
You can have (in theory) infections by applications running in DOS under XP.
I’m just trying to help not to make avast advertisement 8)