Experienced Same issues, Mine hung on Preparing PC, don't turn off computer for over an hour, powered off, and restarted hard, and couldn't open Programs and Features, IE Window wouldn't work, Skype wouldn't work on my 8.1 64bit with Media Center Machine.
Laptop couldn't access for a while Avast forum, so i ended up doing a full hard disk wipe, and starting full reload, Windows 7 64bit, then Windows 8.0, then finally if all goes well 8.1 Install via the app store, only way to do it for me, won't take my key any other way.
Will make room for image after all this on external drive, this is a major pain