For computer users, a few mouse clicks could mean the difference between staying online and losing Internet connections this summer. Unknown to most of them, their problem began when international hackers ran an online advertising scam to take control of infected computers around the world. In a highly unusual response, the FBI set up a safety net months ago using government computers to prevent Internet disruptions for those infected users. But that system is to be shut down. The FBI is encouraging users to visit a website run by its security partner, http://www.dcwg.org , that will inform them whether they're...