That’s an AOL proprietary format. Microsoft recently discovered a security flaw (oh, the irony!) in it and stopped support for the .ART viewer in IE. This page describes how to use AOL to convert the files to a more useable format (you don’t need to sign up with AOL to do this).
Thanks Q.E.D, but it isn’t quite working for me. From the page you linked:
This used to work, but now it doesn’t anymore. IE will no longer open them. That’s why I’m kvetching. I’m guessing I’ll have to download an older version of IE, or something.