I'm using Kaspersky Internet Security.  I downloaded a single mp3 from a legal SITE and scanned the file as i do with everything that i download. 

"2 objects scanned, no threats detected"

While i trust Kaspersky, i've NEVER seen an mp3 come up as 2 files.
Can anyone shed some light on this for me?The file has two data streams.

When you download a file, Internet Explorer and most browsers automatically add a data stream to the file, indicating that it is a downloaded file. For example, if you try to run an application with this stream present, Windows will display the "security warning" and tell you that files from the internet may not be secure, BLAH blah. Basically- it uses this stream to "know" if a file was downloaded.

An Alternate Data Stream is a feature of the NTFS file system that NT-based Windows Operating Systems have used. It basically makes a File have more then one set of data associated with it- the "NORMAL" data is in fact stored in a stream called "$DATA" that will be opened if no specific stream name is specified.

Most programs can open Alternate Streams- notepad, for example, can be told to save a file called "file.txt:hidden." if you open file.txt, you'll find that you cannot see  the text you just saved! In order to view your secret data, use "file.txt:hidden." to open the file again.

If your curious about Alternate Data Streams, I have released a program called "BCStreams" that lets you view the alternate data streams of a file: http://bc-programming.com/index.php?page=downloads.

However, since that is a command-line tool and I have not yet created an easier to use GUI tool for it, you MIGHT prefer to use something like the JSWare StreamViewer, which adds a new tab to the "properties" dialog of any file, listing the alternate data streams that exist in it.