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Reports of Stratio-Zip, Netsky-D and MyDoom-O infections accounted for 39.7 per cent of all malware currently circulating during the month.

Variants of the Stratio worm, also known as Warezov, were the most prevalent strain in November, making up a third of all activity.

Sophos tested this family, as well as Netsky-D, the fifth most prevalent virus with 3.9 per cent, and MyDoom-O - sixth with 2.5 per cent - against the Vista operating system which launches today to the business community.

It found that the Windows Mail client did a good job of identifying and stopping the viruses, which use email as the attack vector.

However, the Microsoft's flagship operating system was not able to stop infections when a user accessed email

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through an online account, such as Gmail, Hotmail and so on.

This could result in businesses having to adopt a draconian policy on employees accessing webmail unless systems are shored up with third party security software.

'There has been much speculation about whether Vista would render existing malware extinct, and the news is now in - it won't,' said Carole Theriault, senior security consultant at Sophos. 'While Microsoft should be commended for the huge security improvements it has made in Vista, running separate security software is still essential to eliminate the risk of infection. On top of this, cyber criminals will already be looking at creating Vista-specific malware. Users need to think carefully about whether their current solution is going to offer sufficient protection against such emerging threats, given that some vendors continue to experience problems adapting their software for the Vista operating environment.'

Sophos and Symantec, among others, have already announced Vista-ready protection in their respective security suites.

For November, Sophos noted the ratio of viruses in email traffic was 0.28 per cent, despite a record month of 7,612 new threats. Managed security vendor MessageLabs, had the figure as 0.7 per cent, saying the level had decreased a quarter of a point sequentially.