Avast used to support Windows Phone which is why I originally started using it. I am baffled as to why it was removed from their product line as there doesn't seem to be any of the well known AV companies supporting it, so could be a niche market with sales opportunities.
I for one love my Windows Phone and will not be ditching it for an android or an iphone anytime soon.
Avast, please please please port your applications to Windows Phone.