Hello,
I just wanted to add my voice to the growing chorus about this issue...because I've been affected, too.
* I just updated/upgraded my Avast Free Antivirus 2015 to v10.3.2223 (just like some others I saw in this thread)
* I run WinXP as well (just like some others I saw in this thread)
* I get peculiar "invalid certificate" errors at some unexpected sites (i.e., google.com, netaddress.com {my email provider} and avast.com{!} are those I tried, and where the error also showed up)
But...
* I called Avast for support (7/26/15, 11:30pm ET, usually GMT-5) and the technician (Benjamin) insisted that this was my computer's fault, and not Avast's for supplying a bad update.
* He claimed to not have heard of this issue before my call (do they NOT read these forums?)
* He then insisted that I'd need to pay for "Tier 2 Support" that he said I needed to fix this.
* He THEN said that I should talk to Microsoft for further assistance because (again) it was my computer (which he kept saying was running "Windows HP", not XP, while besmirching MY computer knowledge as, he insisted, necessarily being less than his -- I have my doubts) causing this as far as he was concerned...even though this thread intimates that it was, indeed, Avast's update, instead, as I suspected.
So, anyway...
I'm posting here so that I can hopefully receive an update of an imminent(?) post from someone authoritative and knowledgeable at Avast with more information. I hope it comes soon...
Thanks,
Bram