Z4, based in Michigan, owns some 24 patents for digital rights management. In April last year a jury in Marshall Texas found that Microsoft had infringed two of the patents in its Windows and Office software.

Although a permanent injunction was denied by the court, following the Supreme Court’s eBay decision, the jury awarded damages of $110 million, which were increased to $140 million. The Federal Circuit upheld this award on November 16.

At that time, Microsoft denied that Windows Vista and Office 2007 were affected by the ruling.

Now it appears that Z4 has sued Microsoft again, claiming that the company has only made “an insignificant change” to the infringing technology in Windows Vista and Office 2007.

According to reports in the US, Z4 also claims that it notified Microsoft of this infringement in December last year.

Microsoft is reported to be reviewing the suit, but claims that Windows Vista and Office 2007 do not infringe Z4’s patents.

Z4, a privately owned company that specializes in researching DRM technologies but is not known as a manufacturer, is viewed by some people as a typical patent troll.

Z4’s attorneys had not returned calls from MIP Week seeking clarification before press time.