The company is accusing the defendants of spreading so-called "malvertisements" - ads that redirect users to sites that infect their computers with damaging software.

Microsoft's associate general counsel, Tim Cranton, announced the move in a blog post.

"Microsoft works with the other leading providers of online ad platforms to mitigate the threat posed by malvertising, but we're now taking that effort a step further," Cranton wrote.

The software maker filed the claims against five unknown individuals operating under the business names Soft Solutions, DirectAd, qiweroqw.com, ITmeter INC. and ote2008.info.

Because their identities are unknown, the court proceedings in Washington State Court are filed against a series of "John Does".

Microsoft is asking a court in Seattle for damages as well as an order barring the advertisers from operating.