CBS Corp., the top-rated U.S. TV network, said it has agreed to buy a 33% stake in WinStar Communications Inc.'s Office.com in exchange for $42 million in advertising, expanding its online holdings. Over six years, CBS will promote and provide advertising for Office.com on its TV network and affiliated stations, as well as across the radio and billboard businesses of Infinity Broadcasting Corp., in which CBS owns an 83% stake. Office.com, which will be fully introduced this summer, supplies editorial content and business tools to Yahoo Inc. sites and will expand to include commerce and communications tools and other information helpful to small and medium-size businesses. WinStar said it plans to keep its name closely tied to Office.com as it aims to expand the WinStar brand. Shares of New York-based WinStar rose $2.13 to close at $46.25 on Nasdaq; CBS shares rose 81 cents to close at $44.38 on the New York Stock Exchange.