Corporate networks that allow Napster downloads are sitting ducks for hackers, says one network security expert. "We call it risky Internet behavior," says Chris Rouland, director of research at Atlanta-based Internet Security Systems Inc., a leading computer security firm. The risk comes from users opening up their computers to anonymous Web surfers who select music files to download to their own computers. Network administrators also have complained about inappropriate use of corporate networks and traffic tie-ups from digital music downloads.