A batch of lawsuits that charge search engines with trademark infringement could upend online advertising, transforming the current system in which competitors jockey to have paid ads appear alongside natural search results. The car insurer Geico, Playboy Enterprises, and American Blind and Wallpaper, a home-furnishings company in Plymouth, Mich., are among the plaintiffs. In its suit, American Blind charges that by directing searches for the phrase American Blinds to ads for its rivals, Google abets infringement. At presstime, a Google search of the firm's name produced natural results -- including American Blind's homepage -- along with 10 sponsored links, six of which belong to competitors. Google beat a similar suit brought by Geico, but Netscape settled with Playboy over banner ads triggered by the trademarked words Playboy and Playmate. Still, American Blind faces an uphill battle, according to Lawrence Townsend, an intellectual-property attorney with Owen, Wickersham & Erickson in San Francisco. Its name, he explains, is just too generic.