Well, now with the screenshot, we can understand that it is NOT Avast starting in some virtual environment or alike.

According to that screenshot, there is some “winampa.exe” trying to run something (or the other way around if you want, “something” is trying to run winampa.exe). Avast is telling you that this “EXACT” and “specific” action is not a very common and expected one, so it offers you the chance to run it only inside the “virtual environment”.

The site that is triggering this action is also in that screenshot.