I had this problem with updating via V7's GUI. Ended up with a reinstall of my Windows 8 installation.
Clearly, something went quite wrong up here.
Can't really remember what it was exactly, but a re-install of Windows wasn't a wrong thing to do anyway.
After installing all basic stuff, I installed version 8 of Avast through the express install.
But now it's installed, from what I can see, it's quite an improvement!
My compliments to the team!