hey i suggest you do a clean installation of avast.
this usually solve the problem you have.
download the latest version of avast from avast.com
download and run the uninstall tool for avast
reboot your system
install the new version of avast you downloaded.
link to the uninstall tool
http://www.avast.com/en-se/uninstall-utility
link to avast program
http://www.avast.com/en-se/index
let us know how it goes.