Microsoft has promised to pay $250,000 (£140,000) to anyone who helps authorities to find and prosecute the author of the fast-spreading computer worm MyDoom.
The internet security experts mi2g have declared MyDoom the second worst piece of malicious software (malware) of all time and estimated the economic damage wreaked on computers in more than 200 countries at $22.6bn.
DK Matai, the executive chairman of mi2g, said: "It is now only a matter of time before MyDoom surpasses Sobig as the most damaging malware ever."
Microsoft's cash reward for MyDoom.B, a more potent variant of the earlier MyDoom.A malware, is the third so far under a $5m programme announced by the software giant in November to help to track down the authors of unusually damaging viruses and worms aimed at consumers of the company's software products.
The MyDoom.B worm - also known as Novarg - was first detected on Wednesday. It infects computers via an email with an attachment carrying the file extension .bat, .cmd, .exe, .pif, .scr, or .zip. The message's subject line also varies, but it can include Hi, Hello, Status, Error, Undeliverable, and Mail transaction failed.
Computer users who open the attachment will open up their computers to further attacks from hackers and automatically send infected emails to everyone in their address book. MyDoom also blocks access to sites of antivirus companies and the Microsoft site. The virus poses as an authentic-looking error message.
MyDoom is also programmed to use infected machines to launch a denial of service attack against Microsoft.com and sco.com, the website of SCO Group, another computer firm, this Sunday, flooding them with useless web traffic so that legitimate users cannot gain access.
"MyDoom has been incredibly fast at achieving this notoriety and its components of piracy, surrogacy, denial of service and associated hazards are yet to come into play. What lies ahead on Sunday and beyond is difficult to predict," mi2g's Mr Matai said.
Brad Smith, Microsoft's senior vice-president and general counsel, has described MyDoom as "a criminal attack".
"Microsoft wants to help the authorities to catch this criminal."
Among the only clues to the identity of the possible author was a mysterious message inside the virus: "Andy; I'm just doing my job, nothing personal, sorry."
Microsoft urged anyone with information about the author of the worm to contact the FBI or Interpol.
The SCO Group had offered a $250,000 reward for information leading to the arrest and conviction of the creator of the Mydoom.A version. Experts have said the same person probably created both versions.
Government officials have described the $250,000 rewards as the highest in recent memory funded entirely by the private sector - akin to cash bounties paid in the late 1800s by Western banks to vigilantes who hunted robbers.
Internal FBI documents, obtained by the Associated Press news agency, indicate the US government is a cautious supporter of Microsoft offering cash bounties.
FBI officials gave conditional approval to Microsoft for the proposal in October. But they cautioned that they will not share secret details of any investigation with Microsoft executives nor necessarily launch an investigation whenever the company announces a reward.
In the documents, obtained under the US Freedom of Information Act, the FBI said it was developing "a more formal operating protocol for working with Microsoft" and with other companies that want to offer similar rewards.
Microsoft said residents of any country are eligible for the $250,000. The company has said previously it will not pay rewards to anyone involved in creating the viruses.
Previous rewards of $250,000 each were offered for information about those responsible for the Blaster and Sobig viruses, which spread rapidly last summer among hundreds of thousands of computers running Windows.
