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, FX-6300 CPU, RX 470 GPU, 8GB RAM, Windows 10 Home 64 bit
HP dv6-6140us laptop, A8-3500M APU, 8GB RAM, Windows 7 Home Premium 64 bit
RCA W101 v2 10" tablet, Intel Atom Bay Trail Z3735F processor, 2GB RAM, Windows 10 Home 32 bit