In the past, virus writers seeking fame and attention wrote their malicious programs to spread as quickly and broadly as possible, boasting to colleagues when they manage to cripple hundreds of thousands of computers worldwide in a matter of hours. But now, many writers are driven by money instead. They write code to turn the computers of unsuspecting individuals into “botnets” — networks for spreading junk e-mail or stealing financial data from others. Security experts find that some are even taking measures to make sure their programs don’t spread too quickly or too broadly, lest they get detected and blocked.