This happened to me, on both my desktop and my laptop. Avast is just not there. I can't find it - it's not there to uninstall, I can't find it by searching icons.
It's as if I never installed the bloody thing. I suppose I can always just get a new copy and reinstall and put my key in there, but it's slightly concerning that an upgrade to 8.1 "wiped out" Avast.
I was using Win8 Pro 64bit on desktop and Win8 Pro 32bit on laptop. The desktop was using Internet Security 8.0.1497 Laptop was using whatever the current free version was, not sure there.
UPDATE: It also appears to no longer be INSTALLED, I couldn't find it on my hard drive anywhere. Currently downloading a new version, but that's somewhat offensive that they would uninstall that, which is what appears to have happened.