Steven Sinofsky, who since 2009 has served as president of Microsoft's Windows and Windows Live divisions, has left the company less than a month after launching what Steve Ballmer called the most important operating system in Redmond's history.

"It is impossible to count the blessings I have received over my years at Microsoft," Sinofsky said in a statement on Monday. "I am humbled by the professionalism and generosity of everyone I have had the good fortune to work with at this awesome company."

Stepping up to lead product development for all future versions of Windows will be Julie Larson-Green, who has led various projects at Microsoft since joining the company in 1993. Most recently, she was responsible for program management, UI design and research, and internationalization for Windows 7 and 8.

In addition to heading up the Windows group, Larson-Green's new role will see her in charge of engineering for future Windows-based hardware products, such as the company's recently launched Surface fondleslabs.

Microsoft CEO Steve Ballmer expressed the utmost confidence in Larson-Green, calling her "the best possible person for this job." Still, the responsibility of running Redmond's most important product divisions won't fall solely on her shoulders.

Bolstering her on the business side of affairs will be Tami Reller, currently chief financial officer and chief marketing officer for the Windows division, who will add a third hat to her already crowded head.

In her newly expanded role, Reller will be responsible for driving business and marketing strategy for Windows and all Windows-powered devices, including those manufactured by Microsoft partners.

Both women will report directly to Ballmer.

The leadership changes come on the heels of a series of significant new product launches for the Windows division, including not just Surface, but more importantly Windows 8 and Windows Phone 8.

Microsoft's news release gave no reason for Sinofsky's departure, but said that it is effective immediately. ®