March was relatively quiet in the virus world, with most researchers reporting few new variants and no widespread outbreaks.

Of the seven monthly reports filed so far, most showed old viruses like Mytob and Netsky still leading the infection charts in a sign that writers are concentrating on targeted Trojan software rather than traditional mass worm outbreaks.

"Downloaders and other Trojans are now de rigueur," said Mark Sunner, chief technology officer at MessageLabs.

"With major advances in technology for detecting mass-mailing malware, coupled with the desire to create an attractive market for Trojans that remain invisible for longer, these are proving to be a real draw, rather than the alternatives that draw attention to themselves by continually sending virus infected emails."

Spam rates dropped by a few percentage points over the month, with most companies reporting a fall. However, online security firm BlackSpider noted a drop in the amount of porn-related spam of 33 per cent.

"The huge drop in porn-related spam is surprising, as this is a 'lowest common denominator' category that obviously works. But it highly likely that porn spam will be back in greater numbers," said James Kay, chief technology officer at BlackSpider.

Sophos, meanwhile, reported that the Hotmail email has lost its top spot in the hoax top 10 after nearly two years. The Olympic torch email, which warns readers that their hard drive is about to be wiped, now has the number one spot.

"The Olympic torch chain letter, which warns recipients that their hard disk will be 'burned' if a certain attachment is opened, has gathered momentum, leaping up the chart to number one," said Graham Cluley, senior technology consultant at Sophos.

"By instilling panic in users about the safety of their computers, the hoaxer has clearly hit on a raw nerve."