Well I also have no issues with the latest version on W8.1 or xpSP3.
imho I feel there are users that are installing Avast on systems that may no be free of other security remnants and this could be the cause of many installation failures.
Security program code goes extremely deep into the OS and it is very reasonable to expect some stuff being left behind if proper "uninstall" procedure is not maintained. I know this from many years of testing different Security software.
Just some added thoughts.