Avast has released another program update for 2014 products with build no 9.0.2008 that has fixed metro apps issues after upgrade to Windows 8.1 and resolves warning shown by Outpost Firewall "unsigned kernel mode driver or service".
You can update to this version from "Update" tab in Avast Settings. You need to restart your computer to apply this update. If you don’t know: all Avast older versions prior to 2014 won’t work or simply gets disabled when you upgrade from Windows 8 to 8.1. So make sure to upgrade to Avast 2014 latest version in Windows 8 before updating to Windows 8.1.
Have you found any issues after updating to latest version? let us know in comments.