Steve Ballmer needs to testify in the Vista Capable class action lawsuit, U.S. District Court Judge Marsha Pechman ruled at the end of last week, giving him 30 days to make his deposition. The decision came despite attempts from Microsoft to stop this from happening, claiming Ballmer had no knowledge of what happened.

Ballmer previously released a statement saying: “I was not involved in any of the operational decisions about the Windows Vista Capable program. I was not involved in establishing the requirements computers must satisfy to qualify for the Windows Vista Capable program. I was no involved in formulating any marketing strategy or any public messaging surrounding the Windows Vista Capable program.”

But according to the judge’s ruling, Ballmer may have unique personal knowledge of relevant facts regarding the case, and therefore his deposition is required.

Microsoft former execs Jim Allchin and Will Poole are accused that in 2006, they agreed to lower Windows Vista requirements after exchanging a series of e-mails with Intel CEO Paul Otellini. That resulted in the Intel 915 chipsets being labeled as Vista Ready, despite lacking the initial requirements that other chipsets producers, such as HP, had qualified for.

The e-mails revealed by the court show how a grateful Paul Otellini thanks Will Poole for the commitment to embrace the 915 chipsets. But the e-mails also show how HP considered the decision unacceptable, accusing Microsoft of bowing to pressure from a partner who needed to get rid of a lot of PCs that were not Vista Ready.

Several Microsoft experts expressed concern over the decision to lower Vista requirements, warning that including the 915 chipsets, which Microsoft had previously excluded from the Vista Capable league, in the Vista Ready program would confuse consumers, who won’t be able to make a difference between a Vista Ready computer and a Vista capable computer.

This resulted in a lawsuit against Microsoft for deceiving consumers in the Vista Capable marketing campaign, and for not mentioning that Windows Vista Home Basic did not include a lot of the features advertised for Vista.

Microsoft denied the allegations, claiming that although the Home Basic version lacked some features found on premium editions of Windows Vista, it was still part of the Vista family and its capabilities have been clearly mentioned.

The company also denied allegations that the Vista Capable marketing campaign resulted in an artificial increase in PC prices and in unfair gains from these sales. Earlier this year, a judge dismissed claims that Microsoft deceived consumers, but the question over the campaign’s price implications still needs to be answered.

According to the court’s latest decision, Microsoft CEO Steve Ballmer will have to share his knowledge on the 2006 situation, after the plaintiffs presented evidence of a thank you e-mail sent by Paul Otellini to Steve Ballmer. The case is set to go on trial in April next year.