Battle Begun on Internet Ad Blocking
Internet users are increasingly turning to software that blocks advertising; software, known by such names as At Guard, Internet Junkbuster Proxy, Intermute and Web Washer, works by blocking ads before they appear on the user's screen; they speed performance, since files that contain ads laden with graphics and animation take far longer to load than files with text; Web-based businesses fear that if this software really catches on, it could clog their revenue stream and challenge fundamental structure of emerging Internet industry; producers of ad-blocking software say demand will grow as advertisers continue to add graphics and sound to their Web pages, making users wait longer for ad-supported pages; sample page with and without ad; graph (M)