If you start to get strange pop-ups (usually porn sites), downloads or other suspicious and odd behavior, you may have one of those. Remember that virus and trojan isn't same thing. Virus is something that infects your computer so you may not even know but trojan is something you have to install and type your admins pass etc. Trojans usually comes with downloaded software like torrents or with plug-ins from porn sites (video doesn't work, download this plug-in = Trojan). I wouldn't worry, risk is low unless you watch porn 24/7 or download illegal software