"The attackers are very opportunistic, and they latch onto any event that might be used to lure people," said Joshua Talbot, a manager with Symantec Security Response. When Symantec tracked the 50 most common malicious programs last year, it found that 56 percent of all attacks included Trojan horse programs.

In enterprises, a social-engineering technique called spearphishing is a serious problem. In spearphishing, the criminals take the time to figure out who they're attacking, and then they create a specially crafted program or a maliciously encoded document that the victim is likely to want to open -- materials from a conference they've attended or a planning document from an organization that they do business with.

With its new SmartScreen Filter Application Reputation screening, introduced in IE 9, Internet Explorer provides a first line of defense against Trojan horse programs, including Trojans sent in spearphishing attacks.

IE also warns users when they're being tricked into visiting malicious websites, another way that social-engineering hackers can infect computer users. In the past two years, IE's SmartScreen has blocked more than 1.5 billion Web and download attacks, according to Jeb Haber, program manager lead for SmartScreen.

Haber agreed that better browser protection is pushing the criminals into social engineering, especially over the past two years. "You're just seeing an explosion in direct attacks on users with social engineering," he said. "We were really surprised by the volumes. The volumes have been crazy."