More than 63% of new malicious programs that appeared during the second quarter of 2008 were Trojans, according to the latest quarterly report from security outfit PandaLabs.
Adware, with 22.40%, was the second most frequently created type of malware in the same period.
Luis Corrons, technical director of PandaLabs, said, "In the present scenario, it is unlikely for a Trojan to infect a large number of computers, as this would attract attention and goes against the interests of cyber-criminals.
"They prefer instead to create numerous different Trojans, targeting users of a specific service or utility, instead of trying to massively propagate a single example."
This is why Trojans consistently figure as the type of malware with most individual examples in circulation, he said.
Trojans were also the type of malware responsible for most infections in the second quarter of 2008, accounting for 28.7% of the total.
Adware, which held first place in the first quarter of the year, was the cause of 22.03% of infections, and worms were the culprits in 13.52% of cases.
Of all types of Trojan in circulation, those designed to target online banks and payment platforms were by far the most dangerous, said PandaLabs. These are known as banker Trojans.
"This type of malware is causing serious losses for users around the world, particularly considering the increased use of online banking services.
"In 2006, in the US alone, there were already 44m online bank users. This is a tremendous pool of potential victims for cyber-crooks," said Corrons.