Hello,
Don't be worried about it. Avast! does its job. And I wanted to add that SPAM and viruses are different things. Of course, SPAM may contain viruses inside in some cases. And if you won't get any virus warning when surfing, it's normalas not all sites hide viruses inside
seems your teenager surfs through good and clean sites
If virus will be found in website, webshield will stop it even before you'll get it in your PC, so good luck using avast, surf safe, and don't be worried, avast will warn you when something goes wrong.