Defender is useful for assuring System Restore points I assumed and doesnt get in the way, but as a shield pretty feeble and clunking for what it does as a scanner (which I always turn off).
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I installed an uninstalled ZA for the fourth and last time. I do not know what kept stopping it in the final stage. It was even blocking internet access until uninstalled.
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An additional Firewall to the MS Vista one seems to be a good idea though, to cover for Avast which cannot cover everything, even though it remains by far the best AV of the frewrae AVs I have tried (Avira and AVG.)
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I note that as an Avast Free user you lay store on a paid firewall, Outpost Firewall Pro. Of the two you suggested, I like the CNET write up on Privatefirewall (assuming no installation issues). Would you recommend that as an additional shield for online booking (and therefore banking)? Not forgetting other popular sites targeted by hackers?