As this is windows 8 then that is all I use
Turn on the PUP detection in Avast if your teen likes downloading things and try to train him in the use of the custom install option, thereby denying all toolbars and nice extra programmes on offer
Also get him to read this http://blog.avast.com/2013/07/09/shady-practices-of-free-download-servers/
Keep MBAM on the system and run it weekly to be sure