In my own experience, Norton and AVG used to slowdown computers dramatically, but this was 7-10 years ago. That made me stay away from them.
I have heard Norton is not longer that heavy on Wndows, but I still have to prove that, but once you have a bad experience with something, you ask yourself, Why use it again when there are many options out there?
I have used ZA for years without issues, but now I want explore other options. May be I am bored because no matter what I run, I get nothing, LOL. Also, having to pay another $40 for 3 PC's makes me check other suites.
Still undecided what to do. For regular users, any AV + Firewall will be ok, but it is not my case.
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