Older worms and viruses continued to dominate March's list of Top 10 baddest apples, said security firms Thursday, in part because users don't update their anti-virus defenses, but also because 2005's entries have been too weak to unseat the old guard.

According to the list produced monthly by Sophos, the Zafi.d worm led the Top 10 for March by accounting for 45.1 percent of all the malicious traffic the U.K.-based security vendor monitored. Netsky.p came in second with 21 percent of the month's total. Rounding out the top 10 were Zafi.b, Sober.k, Netsky.d, Netsky.z, Netsky.b, MyDoom.o, Netsky.c, and Netsky.q.

"The older worms continue to spread insidiously," said Graham Cluley, a senior technology consultant with Sophos. "They're just not dying off, and it's because there are a lot of people who haven't protected their computers.

"Just because your new PC came with anti-virus software, you mustn't think that that's the end of the story. Those trial versions typically expire in a few weeks, and even during the free-use period, they're usually out of date, since they're built on old disk images."

Zafi.d, for instance, first appeared in mid-December 2004, and has held the top spot in Sophos Top 10 since then. All but one of the ten worms or viruses, in fact, are from 2004.

"Think of Typhoid Mary, spreading disease. Unprotected PCs are like that. Similarly, poxed PCs continue to spread these diseases," said Cluley.

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