The 8.1 preview is very buggy with new features like the right click menu of the new Start button that stop working for no reason and simple things like pinning apps to the Start screen not working. Avast should wait until the bugs get ironed out unless they want to play an ongoing process of catch up.
HP6140 Laptop, AMD A8-3500M Quad Core APU, 6GB RAM, Windows 7 Home Premium 64 SP1, IE10 & Outlook 2007, Comodo Internet Security 6.3, MBAM on demand
Google/Asus Nexus 7 tablet, Tegra 3 quad core 1.3ghz, 32GB storage, Android 4.3, Dolphin browser, TrustGo Mobile Security
HP 5215us laptop, AMD Turion64 single core CPU, 2GB RAM, Windows 8.1 pro 32 bit, IE11 & Outlook 2007, Comodo Internet Security 6.3--- All machines behind a NAT router