No !!! Avast is not stable with Windows 8 nor Windows 8.1...
I have tried both. Avast on Windows 8 filling my Event Log with bunch of errors. Avast with Windows 8.1 destroyed my system. After installing it, system went crazy and spammed errors each 3 seconds. I was not able to do delete avast, to do system restore, to run safe mode... I had to do clean install again...