Avast9 works perfectly well with Win8.1.

The issue of apps/programs being removed on update is a Windows problem.

I have had a few programs where this has happened, like the software for my HP all in one was removed, I tried to re install with the latest HP software and Windows prevents it, saying it is incompatible.

HP have written a special “wizard” which finds and installs the software for you on Win8/8.1.