My guess is that you still have XP SP2 ?
The reason I mention this is I use XP Pro SP3 and this issue isn’t present on my system.

This issue relating to untrusted sites is to do with the new ability to scan HTTPS traffic was incompatible with XP SP2. That said avast 10.3.2225 and an earlier hotfix in the virus definitions and engine updates should have disabled this HTTPS scanning in XP SP2 or earlier.

You are no more exposed than before avast introduced the HTTPS scanning function.