Sorry to intrude but I would like to drop my 2 cents.
I deem it impossible to catch a virus from downloading video files off a tracker. The only way you could be catching a virus is because the client software is infected with one (unlikely), make sure you get a solid client, not something shaky made by an amateur programmer.
As far as .torrent files are concerned, they contain nothing except hash and checksums. The .torrent is like a map to get the video store if you like.
On top of that, you can't catch a virus off an .AVI because they dont execute code, they just consist of a container with the video and a sound channel. So I would advise you to check again, where exactly your getting the virus from. It sounds like the client is infected with a virus or some malware that is trying to inject itself into the client...