Security firms are reporting a worrying surge in the amount of new Trojan software circulating on the internet.
For the past few years mass mailing worms have been fashionable among virus writers, particularly those looking to create widespread outbreaks.
But the growth in Trojan detection demonstrates the increasing use of malware to steal personal information.
"It looks like Trojans are a real growth area," said Graham Cluley, senior technology consultant at Sophos.
"They don't tend to show up in virus charts because they don't self-replicate. It's a sign of the times; in the old days virus writers just wanted to make a splash, but now a new group is going after profit."