The threat posed to computer users and companies by hackers is shifting from attacks on the computers to attacks on electronic transactions, according to the head of one of the world’s largest security software vendors. John Thompson, chairman and CEO of Symantec Corp., said the change has been taking place over the past few years but has recently been accelerating. “While a few years ago many people were much more focused on attacking the machine and attacking the broad-based activities that were going on online, now all of a sudden we’ve noticed a significant shift in both the type of attack and the motivation of the attack,” he said.