Microsoft is working with the authorities to find the author of the Sasser worm, which first appeared on Friday and targets Windows XP and 2000.

The software giant is helping the Northwest CyberCrime Taskforce (a joint FBI and US Secret Service organisation) to analyse Sasser's code and "identify those responsible for this malicious activity," the company said in a statement.

Sasser appeared on Friday and exploits a recently disclosed hole in a component of Windows called the Local Security Authority Subsystem Service, or LSASS. Microsoft released a software patch, MS04-011, on 13 April to plug it.

The patch has been downloaded more than 150 million times, said Michael Reavey, security program manager with Microsoft's Security Response Center. Overall, patch downloads have quadrupled since the company first introduced its new Protect Your PC campaign in 2003.

Sasser is similar to an earlier worm, Blaster, because users do not need to receive an e-mail message or open a file to be infected. Instead, just having a vulnerable Windows machine connected to the Internet with communications port number 445 is enough to get infected.

Microsoft issued a statement on Sunday saying that it is working with the Taskforce to analyse malicious code in Sasser and in a Trojan program called Agobot, which was also modified to take advantage of the LSASS hole.

Computer virus experts have noted similarities between computer code in the Sasser worm and a common family of e-mail worms called "Netsky". Reavey could not provide details about those links or about the ongoing investigation into the Sasser author on Monday. "All I can say is that once we find him, we're going to make sure he's prosecuted," Reavey said.

Microsoft referred questions about the investigation to the task force. Calls to the FBI, the Secret Service and the Seattle police were not immediately returned.

The company also announced other steps it is taking to lessen the damage caused by Sasser, estimated to have infected hundreds of thousands of Windows XP and Windows 2000 machines on the Internet.

Microsoft released a free software program to clean Windows systems infected with Sasser. The company also published information on how to configure firewalls to stop the worm's spread and encouraged customers to enable their personal firewalls and install the Microsoft Windows patch that fixes the vulnerability Sasser exploits. The company will continue to update the tool to work with new variants of the Sasser worm, Reavey said.

Besides working with law enforcement, Microsoft also offers bounties for information leading to the arrest of those responsible for major viruses and worms. In November, the company announced it was allocating $5 million to a reward fund for the arrest of virus authors. In January, the company said that it was offering a $250,000 reward for information leading to the arrest and conviction of the author of the MyDoom.B worm.