Microsoft Corp., weary of being cast as a stodgy oldster by Apple Inc.'s advertising, is turning for help to Jerry Seinfeld.

The software giant's new $300 million advertising campaign, devised by a newly hired ad agency, has been closely guarded.

But Mr. Seinfeld will be one of the key celebrity pitchmen, say people close to the situation. He will appear with Microsoft Chairman Bill Gates in ads and receive about $10 million for the work, they say.

The new ad effort is expected to use some variation of the slogan "Windows, Not Walls," according to several people familiar with the matter. Those people say the point is to stress breaking down barriers that prevent people and ideas from connecting.

The campaign, said to debut Sept. 4, is one of the largest in the company's history.

The attempted image overhaul comes as Microsoft executives privately acknowledge that Windows — the company's most important brand — has grown stale and has been battered by Apple's "Mac vs. PC" ads.

Those ads, created by Omnicom Group Inc.'s TBWA/Chiat/Day, feature a nerdy PC guy getting upstaged by a hip Mac counterpart.

Microsoft's immediate goal is to reverse the negative public perception of Windows Vista, the latest version of the company's personal-computer operating system.

Windows is Microsoft's largest generator of profit and revenue, accounting for 28 percent of the company's revenue of $60.4 billion in the year ended June 30.

The software has sold well, and Microsoft retains an overwhelming share of the market for operating system software over Apple.

But Apple's computer sales have been rising, and Vista is dogged by the notion that it has technical shortcomings and is hard to use. Apple's latest Mac vs. PC ads take swipes at Vista.

Microsoft says early problems with Vista have been largely alleviated.

•Click here to read more in the Wall Street Journal.

• Click here to watch Apple's 'Get a Mac' ads.