Microsoft chief executive Steve Ballmer has failed in his attempt to avoid testifying in the Vista Capable class-action lawsuit.

Ballmer had issued a statement saying that he was not involved in the campaign, naming two other Microsoft managers as the prime movers, both of whom have since left the company.

However, Judge Marsha Pechman said in a court ruling (PDF) that Ballmer must testify within the next 30 days over his involvement in the marketing campaign, saying that documents released by Microsoft showed that Ballmer had spoken to Intel chief executive Paul Otellini about the campaign.

"Plaintiffs have met their burden in demonstrating Mr. Ballmer may have relevant, unique personal knowledge of relevant facts," she said.

"Microsoft concedes Mr Ballmer discussed the technical requirements of Vista Capable with Mr Otellini and relayed those concerns to other members of the management. The court appreciates that there are severe demands on Mr Ballmer's time; however, a busy schedule cannot 'shield' an executive from discovery."

The ruling is a blow to Microsoft, which had pleaded earlier that the case be denied class-action status and dropped.

If the court finds against Microsoft the financial hit to the company could be huge. Businesses and individuals who bought 'Vista Capable' hardware could be eligible for damages up to the cost of the equipment, and Microsoft would also face a possible fine.