September's Top 10 list of viruses and worms sport months-old malware in the first and second spots, a security firm announced this week, a sign that businesses and consumers aren't updating their anti-virus defenses.

At the top of Sophos' monthly list were Zafi.b and Netsky.p, worms that debuted in June and March of this year, respectively. Together, the pair -- which have held down the top two spots for three months running -- account for over 57 percent of all the viruses that Sophos detected during September.

"The percentage of reports attributed to Zafi.b has gradually declined over the last three months, but it's still a force to be reckoned with," said Sophos security consultant Carole Theriault in a statement. "[But] the percentage of reports of Netsky.p is actually growing. Even when viruses aren't hitting the headlines, it's essential that computer users update their anti-virus protection."

Old viruses continue to cause problems because users don't update like they should. "It's disheartening that the same old viruses are continuing to cause trouble," said Theriault. "These viruses will only disappear when they stop infecting and spreading. Keeping your defenses up to prevent infections is the only way we'll see the back of these nasties."

One bright spot in the month's numbers was that the percentage of e-mail messages infected with a virus was down almost a third compared to August and July.