Microsoft is working with law enforcement to find the author of the Sasser worm, which targets machines running the company's Windows operating system.

The company says it is working closely in an ongoing investigation with authorities, including the Northwest CyberCrime Taskforce to analyse Sasser's code and "identify those responsible for this malicious activity.".

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 April 13 that plugs the LSASS hole.

Customers have downloaded more than 150 million copies of that patch since then, says 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 over the weekend 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 vulnerability.

The task force is a joint effort of the FBI, the US Secret Service, and local law enforcement in Washington state, where Microsoft is based.

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 says.

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.

Among them are a free software program to clean Windows systems infected with the virus. 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 says.

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 $5m (£2.8m) to a reward fund for the arrest of virus authors.