I think I have a similar issue. Over the last few months, if I leave Firefox running, but have left my computer for a few hours, when I return I cannot open a new website. Looking at Taskmanager, I find each time that AvastSvc.exe has taken over 50% of my CPU - and has 100% usage of one of my 2 cores. I cannot end the process (not allowed even tho account has admin privs). The computer becomes unusable and I cannot shutdown properly - if I leave it running the system hangs for hours - finally I have to use the power switch to force a hard shutdown. Lately this has been happening most days. I have used Avast Free AntiVirus very happily on this old XP machine until this behavior started recently.
Info:
Windows XP Pro SP3
Firefox 40.0.3 and earlier
Avast Free AntiVirus 10.4.2233 151010-1
Intel Core 2 CPU 1.86 GHz
EDIT:
Restore Factory Defaults (under Troubleshooting) did not fix the problem.