Microsoft is using offbeat ads to take on a market leader—in this case, Google, which has a commanding lead in the key search advertising market. It attracted 45 percent of all U.S. searches in September, versus 12 percent for Microsoft sites.

Ms. Dewey is the product of Microsoft's internal "buzz" group, which looks to build word-of-mouth appeal. "We try to find new experiences to reach people immune to traditional marketing," said Sean Carver, a group product manager at Microsoft.

Visitors to msdewey.com can ask the snarky vixen any number of questions, just as they would a search engine. But Ms. Dewey responds with some ninja moves and not a little sass. Then, MSN Search results appear to the side.

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