I am having the same problem when I'm using Incredimail. If Incredimail is started, cpu usage is 100%, with AvastSvc.exe in top of the list. If I close Incredimail (completely, not only main window), cpu usage drops to normal. This problem appeared after updating to the last version of Avast: 2015.10.0.2028. I'm using it on a PC with Win XP x64 SP3.