You are wrong that Norton find false positives, I think. Specifically what is it falsely triggering on? Norton is not only faster than your AVAST but is more accurate and updated more frequently wit real data, not stuff that looks like new data.
Any scan that takes 20 hours to complete is completely unacceptable and is not Norton and/or is not a single hard drive. Even including my two terabyte externals, Norton never approaches 20 hours to do a scan. You might wish to run any AV you have, plus a slew of malware detectors. I'm not trying to be as negative as this might sound; I am only trying to point out some food for thought. Even in its "bloat" days Norton never took any 20 hours to scan a hard drive. I've been with Norton since the days of DOS where if the machine wasn't up to the "preferred" system requirements is the only time you'd notce the "bloat". Fortunately, Norton/Symantec listened to its customers and those problems are long gone. My only complaint about Norton is the cost of their subscriptions has gone too high IMO and that does have me contemplating some of the competition sans McAfee, the only one I've fully tested to date.
HTH,
Twayne`