Kevin Johnson, the head of Microsoft's key Windows and Internet unit, is leaving the company to become CEO of Juniper Networks, Microsoft disclosed late Wednesday.

Johnson, whose official title is Platforms and Services division president, will depart the company after working "to ensure a smooth transition," Microsoft said in a statement. Juniper said it expects Johnson, 47, to arrive in September. He will replace Scott Kriens as Juniper's chief executive. Kriens remains chairman of the networking vendor.

Microsoft said it won't replace Johnson. Instead, it's splitting Platforms and Services into two groups that will report directly to CEO Steve Ballmer. The Windows side will be led by Steven Sinofsky, Jon DeVaan, and Bill Veghte, all of whom are technicians. Johnson built his career in sales and marketing roles. Microsoft said it would conduct a search for an individual to lead the online services group.

Johnson's departure may be tied to Microsoft's failed effort to acquire Yahoo. As head of the company's Web efforts, Johnson was intimately involved in the campaign and his portfolio would have expanded considerably had the deal occurred. In its absence, Johnson may have perceived a brighter future at Juniper as Microsoft has struggled to grow its Internet business internally.

Microsoft's shakeup of its Platforms unit also comes at a time when the company's core Windows franchise is under pressure. The latest edition, Windows Vista, has drawn harsh criticism from industry pundits, who point to its heavy system requirements, intrusive security features and lack of compatibility with older software. Few large enterprises have adopted Vista.

Johnson's resignation marks the latest in a series of executive departures at Microsoft.

Joanne Bradford, who was chief media officer for the company's MSN Media Network, resigned in March to join advertising startup Spot Runner. Bradford had also previously served as Microsoft's VP for sales and marketing and as chief media revenue officer.

Microsoft in January confirmed that Rob Short, corporate vice president for Windows Core Technology, had quit. Short, a nineteen-year Microsoft veteran, led the team responsible for designing, developing and testing core components of the Windows operating system.

Microsoft Business division president Jeff Raikes announced his retirement this year and company chairman Bill Gates relinquished his full-time duties last month.