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.