This is a windows problem and is not related to Avast… Check the MS insider forum and there is a lot there since Cortana was released a few months ago.
The MS answer that works is reboot to safe mode then back to normal mode and it has gone
http://answers.microsoft.com/en-us/insider/forum/insider_wintp-insider_cortana/cortana-crashing-w-start-menu-in-build-10074/e4639cce-0a21-4c4b-b79f-18bb8055319e?auth=1 http://www.filecritic.com/how-to-fix-the-start-menu-cortana-critical-error-in-windows-10/ http://www.windows10forums.com/threads/critical-error-start-menu-and-cortana-arent-working.2002/
Plus many many more