I've found Norton (when set to 'On access protection' detects virii before the file even completes loading - and deletes it.

There are a few files out there that link to a site that invites you to "Purchase" the required Codec.... obviously forget it, don't and delete the file from the network.

As a note there seems to be a lot of T3 connections offering files of 851kb or thereabouts (often .zip files), all under different names, all infected with trojans. - Your AV software should stop the download .... just be careful ..... as always on the internet!