> For example, sites like NotDoppler have this
> protection where you can crawl
> <http://i.notdoppler.com/files/strikeforceheroes2.swf>
> , though when you try to spider
> <http://i.notdoppler.com/files/> or
> <http://i.notdoppler.com>, httrack returns | Error: 
> "Not Found" (404) at link i.notdoppler.com/

This is not an anti-website copier protection per se ; just a
security/configuration setting on the web server which prevent from listing
directories. There is no way to "bypass" this, unless links to files in the
directory exist otherwise in html pages.