A roundup of the February virus and spam statistics has shown that Netsky is still the most common virus on the internet, despite all major software vendors having a signature file available for over a year.

Sophos, Fortinet and SoftScan all put Netsky at the top of their monthly chart, and the virus accounted for an average of 18 per cent of all virus detections.

Mytob has also proved commonplace, according to Kaspersky Lab, accounting for a third of all viruses detected last month.

"Netsky does not have an expiration date, nor does it conform to the tactic of staying under the antivirus vendors' radars, which has no doubt contributed to its longevity in the top five," said Diego d'Ambra, chief technical officer at SoftScan.

"However, more worrying is that its apparent staying power is probably due to unprotected computers.

"Compared to other viruses the number of Netsky infected machines is remarkably high considering that the last outbreak was in 2004, plenty of time for users to have cleaned up."

The Clagger Trojan also made an appearance in several lists, despite only appearing last weekend. One estimate suggests that 3.2 million UK inboxes got a copy, and the malware caused something of a storm in the antivirus community after some companies were accused of being slow to detect it. 

"In order for this Clagger Trojan to make an appearance in the top 10, it must have been spammed out to millions and millions of email addresses worldwide," said Carole Theriault, senior security consultant at Sophos.

"Trojan horses, which cannot spread on their own, account for roughly two-thirds of all reported malware. Rather than mass bombardment, most Trojan creators focus on small targeted groups to pilfer cash and sensitive information."

Despite all this activity, it was a quieter month for some security threats. Email monitoring firm MessageLabs reported that spam levels fell to just over 60 per cent of all emails, a five point drop on last month and a long way down from a peak of 94 per cent in July 2004.

Kaspersky also reported a quieter month as more and more people got the message about updating antivirus software. The company noted that Sober, the top worm of 2005, has dropped off the virus charts completely for the first time.