An antivirus service used by tens of thousands of businesses and millions of home users shut down an untold number of computers around the world after it mistakenly identified core parts of Microsoft Windows as threats, the company confirmed Tuesday. Webroot Inc. of Broomfield, Colorado, said it issued an updated detection rule that "identified false positives" for critical Windows operating files Monday afternoon, resulting in those files' being "quarantined" and inaccessible to Windows. Kristin Miller, a spokeswoman for Webroot, said the program incorrectly classified as "bad" a common folder that is often targeted for malware. She said that the false...