Microsoft Corp. and federal law enforcement agents seized computer equipment from Internet hosting facilities across the U.S. in a sweeping legal attack designed to cripple the leading source of junk email on the Internet.

Microsoft launched the raids as part of a civil lawsuit filed in federal court in Seattle in early February against unnamed operators of the Rustock "botnet," a vast network of computers around the globe infected with malicious software that allows its masterminds to distribute enormous volumes of spam,...