Consumers that purchased copies of Windows Vista under Microsoft's Family Discount plan are reporting that they are unable to activate the operating system because of faulty registration keys.

According to numerous blog posts, users attempting to install copies of the Home Premium edition of Windows Vista purchased through the discount program got error messages indicating that the registration keys they received from Microsoft are invalid.

"I got two invalid keys," wrote a poster on technology Web site Neowin.net, adding that "I'm not the only one." Posters on numerous other tech blogs are reporting the same problem.

Under the Family Discount program, consumers who purchase a copy of Windows Vista Ultimate are supposed to be able to upgrade two additional PCs to Windows Home Premium at 60% off the retail price. The upgrade must be done using the Vista Ultimate DVD and registration keys e-mailed by Microsoft.

One blog poster said he received the following response from Microsoft's tech support unit: "We apologize for any issues you are experiencing with the Vista Family Discount license keys. Please be assured that we are aware of these errors and are currently escalating this issue to work toward a resolution. We appreciate your patience in this matter."

Officials from Microsoft weren't immediately available for comment. The company launched the retail version of Windows Vista earlier this week amid much fanfare.