Tara Walpert Levy is changing the way we watch television . . . ads, that is. As president of Visible World, she has tweaked the directmail model to help advertisers target audiences, down to particular households. "You and I will be watching the same show, but I have 8-month-old twins, so maybe I'll see a Pampers ad," she says. "If you're 30 and single, you might get a travel ad." Levy's data suppliers link information about demographics and purchase habits to set-top boxes but do not identify individual consumers. The result: ads that seem relevant but not uncomfortably so.