By Maggie Shiels
Technology reporter, BBC News, Silicon Valley
Waledac infections around the world in a recent 24 hour period
Security experts are split over the effectiveness of Microsoft's efforts to shut down a network of PCs that could send 1.5 billion spam messages a day.
The firm persuaded a US judge to issue a court order to cripple 277 internet domains used by the Waledac botnet.
Botnets are usually armies of hijacked Windows PCs that send spam or malware.
"We aim to be more proactive in going after botnets to help protect the internet," said Richard Boscovich, the head of Microsoft's digital crime unit.
"We will do whatever it takes to look out for our customers and our brand. We hope it will spur similar actions," Mr Boscovich told the BBC.
Many saw Waledac as a devastatingly active botnet. Microsoft cited one 18-day period in December when the botnet sent more than 650 million spam e-mails to Hotmail accounts for everything from online pharmacies to fake designer goods, jobs and more.
"This was a worldwide problem and we scored a big, big victory," said Mr Boscovich.
Despite Microsoft's bullishness, some security industry figures were sceptical saying Waledac did not make a big contribution to global spam levels. Others were more impressed with the shutdown.
"Microsoft has taken a bold move in addressing this problem and it will be interesting to see how it develops," said Sandra Toms LaPedis, general manager of the RSA Conference, the world's biggest gathering of security professionals.
To get Waledac shut down, Microsoft said it employed "an innovative application of a tried and true legal strategy".
The court hearing at which the order to sever the domains was issued took place behind closed doors, granted under seal and ex-parte meaning Waledac administrators did not have to attend. In civil cases this is seen as unusual as courts are reluctant to rule without everyone in attendance.
Mr Boscovich said this was the only way it could ensure the success of what it dubbed Operation b49.
"If the organisation behind Waledac had any idea we were trying to shut them down like this, they would have moved their operation.
"But we convinced the court that Microsoft's customers and those of other companies worldwide were suffering irreparable damage and that there was a high probability that evidence would be destroyed," said Mr Boscovich.
He also said that Microsoft's approach was a "unique way to solve a 21st century problem".
The software giant claimed its approach had "quickly and effectively cut off traffic to Waledac and severed the connection to most of its thousands of zombie computers".
Others in the security industry were unconvinced.
Few owners of hijacked machines know they have been compromised
"This will not make the problem disappear. It is a temporary reprieve," Amichai Shulman, chief technology officer for security vendor Imperva, told BBC News.
"In the short term other gangs will fill the void while the people behind Waledac regroup and start their operations all over again."
Security expert Jose Nazario of cybersecurity company Arbor Networks told the Wall Street Journal that the internet addresses Microsoft has brought down could be a small percentage of those used by hackers to control the network.
"The botnet will survive in many cases," said Mr Nazario.
Some in the industry have claimed Waledac was not a prolific spammer and that the effect of its take-down will be minimal.
"If this did affect spam, we haven't noticed," Richard Cox, the chief information officer at anti-spam service Spamhaus told ComputerWorld.com.
"Waledac was not a high threat, it's less than 1% of spam traffic."
Microsoft said although it had effectively shut down the network, thousands of computers would still be infected with malware and advised users to run anti-virus software and keep up to date with security patches.
Security firm Symantec has estimated that over 80% of unsolicited e-mail comes from botnets.