I'll be honest, this one puzzled me and I came up with some really hackneyed solutions to manage the situation where a user-uploaded file contained a virus (I develop lots of community sites which allow file uploads). Most of the solutions I came up with relied on using a FileSystemWatcher running as a service that detected when the Visrus Scanner on the server moved a file to quarantine - which works, but the fact is that even for a short period you have a virus laden file on your Web Server - sysadmins hate that! So, I was doing a little digging around and discovered this, it's basically a virus scanner which works as an ISAPI filter, monitoring all traffic passing into and out of IIS for viruses...probably really old news to most but I though it was cool!