U.S. computer security experts didn’t have much of an Independence Day holiday this year, thanks to a massive botnet-driven attack launched on July 4 that initially targeted several federal agency Web sites and then went wider to include assaults on private sector Web entities in America and sites for South Korean government departments. The denial-of-service attacks, which reportedly began Saturday and were still hammering away as of Tuesday, shut down or slowed response times for several government Web sites.