Malware, including viruses, worms and Trojans, cost global businesses between $169bn and $204bn last year, making it the worst year on record by a wide margin, newly published research has claimed.

According to digital risk management firm mi2g, malware in 2003 did not account for even half of the economic damage sustained in 2004.

The firm estimated that, with around 600 million Windows-based computers worldwide, this works out at between $281 to $340 worth of damage per machine.

"With the latest proliferation of Bagle malware variants worldwide, computer users may be forgiven for having a sense of déjà vu because they have been there before," said mi2g in a statement.

"There was a global epidemic underway at the same time last year with the MyDoom, Netsky and Bagle variants causing significant productivity losses across the globe.

"Windows computers in over 200 countries were infected. Judging by events which unfolded between January and April 2004, there could be a choppy cyber-sea ahead, made all the more complex by new and more dangerous malware families yet to emerge."

The top 10 malware programs of all time, according to mi2g, are MyDoom, Netsky, Sobig, Klez, Sasser, Mimial, Yaha, Swen, Love Bug and Bagle.

DK Matai, executive chairman at mi2g, said: "Malware is getting more sophisticated. The infected emails have different headings, texts and name tags on the attached files, which makes it difficult to identify malware carrying messages for the lay users.

"There is a problem with the present computing paradigm which is the underlying cause for this constant barrage of epidemics.

"It serves the purpose of the vendors to blame the users or the virus writers and not themselves for designing 'Swiss cheese' software."

The economic cost of malware, as calculated by mi2g, factors in helpdesk support costs, overtime payments, contingency outsourcing, loss of business, bandwidth clogging, productivity erosion, management time reallocation, cost of recovery and software upgrades.

When available, intellectual property rights violations as well as customer and supplier liability costs have also been included in the estimates.