I have both installed but I'm still afraid to install the big one.
Really, while the suggestion to create a disk image prior to major changes is certainly good, the recent Avast patch really has solved the problems, at least on my own Win 8.1 computer. Provided you have the latest aswsnx.sys, found at "C:\Windows\System32\drivers\aswsnx.sys", and it shows version 10.0.2208.772, your're good to go with KB3000850 "the big one". Having had my computer borked, my personal preference was to uninstall both Avast & KB3000850 in Win 8.1' safe mode, then reinstall KB3000850 and subsequently Avast, by then updated. But I'm ultra-paranoid! I cannot quite understand why you found it necessary to reisntall your computer from scratch, when the solution was much simpler, but I expect you had valid reasons.