Several Internet worms that have besieged computers for over a week played havoc again, including one called Sobig.F, whose aim was to turn PCs into spam machines and was believed to be the fastest-growing virus ever, experts said. Sobig.F drops software onto infected Windows computers that open them to be used later for distributing Internet spam -- unwanted e-mails and product promotions, experts said. It also represents a new trend in converging e-mail spamming and virus software writing, they said. "We believe [Sobig.F] has been written by a spammer or spammers" looking for ways to get past spam filters,...