Avast Antivirus blocking my site

Hi everybody,

my own site is be blocked by my free avast antivirus. When I want to log in, apears a warning “Trojan blocked” and the site can not be shown. When I call my site, the same thing. When I deactivate Avast, I can get in.
So, is it posible, that someone has put a Trojan in my site? And how can I find and remove it? And how can I prevent that this happens again? Are there good plugins for this work?