Although torrents are widely used for downloading pirated and illegal material, this method of downloading is also widely used for legit software and media such as Linux Distributions so strictly speaking it can be discussed here.
Out of all the three major freebie antivirus programs, Antivir is the best without a doubt so first thing to do is uninstall Avast then download and install Antivir (www.free-av.com).
How risky is downloading torrents? Going in for the naughty stuff will put you at risk, staying this side of the law mostly will not.
Couple of tips, if you're using Windows, go to Folder Options/View and enable all hidden folders and file extensions. And then broadly speaking if you start something with an *.exe file extension then you're taking a risk.
Opening something with, say, an mp3; mp4a; wmg; wma; mpg; avi or wav extension generally won't put you at risk.
But, as ever, if it's not trusted, then it usually means it's illegal so when a nasty completely floors your machine, it's almost a rough justice