As the title already explains, .NET Framework 3.5 installation fails on Windows 8.1 Update 1 64bit with avast! enabled.
You just get the error attached in attachement. After disabling avast! (for 10 minutes option), installation goes through without a problem. Would be nice if someone checked this out, because this shouldn't be happening and there is tons of people having problems with this one and they don't have a clue what's causing it.