"We disagree with the recent statements made by Microsoft on the topic of Linux and patents," Novell CEO Ron Hovsepian wrote in an posted on Novell's site. "Importantly, our agreement with Microsoft is in no way an acknowledgment that Linux infringes upon any Microsoft intellectual property. To claim otherwise is to further sow fear, uncertainty and doubt, and does not offer a fair basis for competition."

A once-bitter rival of Microsoft and creator of the SUSE Linux distribution that competes with Windows, Novell signed an agreement with Microsoft on Nov. 2 in which both companies pledged to make their software work together better, help each other with sales and marketing and protect their corporate customers against possible patent lawsuits.

Microsoft also released a on Monday, saying that it "respects Novell's point of view on the patent issue, even while we respectfully take a different view. Novell is absolutely right in stating that it did not admit or acknowledge any patent problems as part of entering into the patent collaboration agreement. At Microsoft, we undertook our own analysis of our patent portfolio and concluded that it was necessary and important to create a patent covenant for customers of these products."

When the Nov. 2 pact was announced, Microsoft executives declined to say whether they felt the Linux OS includes Microsoft intellectual property or otherwise infringes on its patents.

But in a question-and-answer session at a database conference in Seattle on Thursday, Ballmer openly asserted that "Linux uses our intellectual property" and users of the open-source operating system face "an undisclosed balance-sheet liability" as a result.