Just a thought (welcome here by the way) if you can, download the set-up file for avast again. Once you’ve done that dis-connect from the internet by removing the cable from your modem so you won’t be connecting without any anti-virus software. Delete avast from add/remove programmes and re-install it after a reboot.

As an aside, do you use windowblinds or any other software to enhance the effects in windows?

I’m no expert here, just some things I would try first.