To me this sounds like a file corruption although whether Avast or Windows I would not like to say

A fresh clean install of Avast would help to resolve the area of suspicion.

Download Rejzor’s Uninstall Utility to your Desktop.
Download the correct version of Avast
Avast Free
Avast Pro
Avast Internet Security
Avast Premier
Disconnect from the net
Uninstall Avast via control panel

[]Unzip Rejzor’s tool to the desktop
[
]Run the tool and accept the usage
[]Once complete reboot your system
[
]Reinstall Avast