Although protection has been available for more than three years, the Netsky family of worms still accounted for almost a third of all malware detected during the month of March, 2007.
According to Sophos, the Netsky family had the biggest impact on computer users last month.
Interestingly, just 0.18 percent - or one in 555 of all email - was infected in March, yet Sophos detected 8,835 new threats, bringing the total to 231,548.
Sophos senior security consultant, Carole Theriault, said these numbers indicate that while malware spreading via email is still causing trouble, the vectors used to distribute threats are changing.
"Hackers are continuing their move away from mass-mailing worms in favour of using spam messages with links pointing to infected Web pages," she said.
"Unwanted emails hiding copies of Netsky are still spreading like weeds in an untended garden, showing how well seeded these mass-mailing threats are.
"It's frustrating to think that there are a bunch of new threats out there that are much more targeted and devious in their approach, yet how can we expect the average computer user to protect against them when the Netskys and Mytobs remain so rooted?"
Theriault said users need to roll up their sleeves and commit to keeping their PCs secure.
According to the top 10 list of countries hosting malware-infected Web sites in March, China came in first with 35.6 percent, followed by the United States with 32.3 percent and Germany with 7.5 percent. Australia ranked 34th.