Microsoft Corp. CEO Steve Ballmer has been ordered by a federal judge to testify in a class-action lawsuit, challenging the company's marketing of its Windows Vista operating system.

Earlier this year, S. District Judge Marsha Pechman ruled that consumers could follow a class action suit against Microsoft for labeling some PCs as "Vista Capable," even though many were not powerful enough to run all of Vista's features. 

However, the motion to depose Ballmer has been opposed by Microsoft. On Friday, Pechman wrote that it has been shown by the plaintiffs that Ballmer might have had unique knowledge of the "Vista Capable" program. Within the next 30 days, a deposition must be given by Ballmer. It was confirmed by a company’s spokesman that Microsoft will obey the order.