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.
Avatar FX6327X Desktop, AMD FX-6300 6 core CPU, Radeon R9-270X graphics, 8GB RAM, Windows 8.1 64 bit, Bitdefender Free antivirus, MBAM on demand
HP6140 Laptop, AMD A8-3500M Quad Core APU, 6GB RAM, Windows 7 Home Premium 64 SP1, Bitdefender Free, MBAM
Google/Asus Nexus 7 tablet, Qualcomm Snapdragon quad core 1.5ghz, 32GB storage, Android 5.0.2, Dolphin browser, Bitdefender Free All machines behind a NAT router