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Johnson is president of Microsoft's largest business division, platforms and services, which will be split into two groups following his departure.

The new groups will focus on the Windows operating system and search and online services, respectively.

Three executives working in the Windows division will report directly to Ballmer, while the company says it will

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conduct a search to find a new head for its online services business.

Johnson joined Microsoft in 1992 and worked his way up to being the head of worldwide sales. He was then put in charge of the Windows business and tasked with incorporating the online services business with its traditional software groups.

A source close to Microsoft said that Johnson has been searching for an opportunity to run his own company.

Johnson's departure is another sign of a changing of the guard within Microsoft's top ranks. Bill Gates, Microsoft's non-executive chairman, stepped down from day-to-day duties at the company at the end of June to focus on philanthropy.

Jeff Raikes, who had led Microsoft's Office business, announced earlier this year that he also planned to leave the company. He later accepted the CEO job at Gates' charitable organization, the Bill & Melinda Gates Foundation.

Microsoft did not announce when Johnson would leave.