Having same problem on two of my home computers running Avast Internet Security (program version 2014.9.0.2021). Tried uninstall, CCleaner to clean up registry followed by fresh re-install. Computer ran great for about 30 minutes or so, then AvastSvc.exe jumped up to the top of CPU usage again consuming 99% and bringing the machine to a standstill. Tried disabling SSL email scanning as suggested elsewhere in this post, but that did not work. The only thing that consistently seems to cure it is to disable the firewall, which is not something desirable. One additional point of note is that if I re-enable the firewall, the computer runs fine for about 30 minutes again, but then AvastSvc.exe suddenly jumps to the top CPU user position and stays there.
It's a real shame because Avast! used to be such a good antivirus with low CPU overhead and no pop-up ads. It's most frustrating when software I paid for now assaults me with pop-up ads and cripples my system!