SEATTLE (Reuters) - Microsoft Corp. launched a 15-month advertising campaign on Monday to boost sales of the Windows XP operating system, while the world's largest software maker prepares its next version of Windows, code-named Longhorn.

Redmond, Washington-based Microsoft declined to say how much it would spend on the global television, print and online marketing effort, or how the duration of the campaign might reflect the delivery schedule of Longhorn.

Advertising trade report Adweek estimated the cost of the campaign at about $100 million, including production of more than 50 versions of television commercials for 11 countries. Advertising agency McCann Erickson, part of Interpublic Group of Cos. Inc. (down $0.06 to $12.94, Research), created the campaign.

Scott Lennard, director of advertising for Microsoft's consumer marketing group, said that Microsoft wanted people to "rediscover" Windows XP, currently its flagship operating system, which runs on more than nine out of 10 personal computers.

The massive ad campaign comes three years after the debut of Windows XP and urges people to use the programs to pursue any of more than a dozen different interests, including music, science, art and sports. In the main print ads, a window is displayed over a person's heart, with various images connected to the themes flowing out of the window.

Microsoft's renewed advertising effort also comes as Apple Computer Inc. (up $0.16 to $35.51, Research) prepares to ship the latest operating system for its Macintosh PC later this month.

Apple's software, which it calls "Mac OS X Tiger," is being described by the Cupertino, California-based company as a feature-rich operating system that already delivers many of the features promised by Longhorn.

Microsoft is expected to talk about Longhorn in greater detail next week at its annual hardware and engineering conference in Seattle. Microsoft is planning to release a preliminary test, or beta, version of Longhorn this summer.

Shares of Microsoft (up $0.19 to $24.65, Research) rose .9 percent in afternoon trading on the New York Stock Exchange.

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