An adware program has topped a security firm's monthly malware breakdown for the first time.

Kaspersky Lab believes that the proliferation of the Virtumonde.gen family of programs has been assisted by Trojan downloaders which, together with other types of Trojan, continue to dominate the overall threat landscape.

Kaspersky's Online Scanner chart for February 2008 includes 11 new entries, and half of the chart is occupied by Trojan variants such as diallers, droppers and downloaders.

The prevalence of hybrid malware, in which Trojans are used as a back-door for malicious programs, has continued, according to the report.

Kaspersky recorded several Trojan downloaders that have installed the notoriously hard to remove Virtumonde program on victims' computers.

David Emm, senior technology consultant at Kaspersky Lab, said: "Trojan downloaders that go undetected enable cyber-criminals to update malware on already compromised machines and so extend their 'shelf-life'."

Email worm Win32.NetSky.q topped Kaspersky's virus top 20 for a third successive month. However, Trojan-Downloader.Win32.Small.hsl reached fifth place and Trojan-Downloader.Win32.Diehard.ez reached 12th place.

Aleks Gostev, a virus analyst at Kaspersky Lab, said: "Diehard is continuing to cause significant outbreaks and the number of programs in this family continued to rise in February.

"If the percentages for all variants of this Trojan are added together, in percentage terms Diehard ranked higher than Trojan.Win32.Dialer.yz."