The browser that set off the latest dispute has been in development for some time, but Microsoft first made it available to the public for downloading last week in a test version. It is the first new release of Microsoft's browser in five years. A final version is expected to be released this summer and will be included in Microsoft's new operating system, Windows Vista, which is scheduled for release next January.

The focus of Google's concern is a slender box in the corner of the browser window that allows users to start a search directly instead of first going to the Web site of a search engine like Google, Yahoo or MSN. Typing a query and hitting "Enter" immediately brings up a page of results from a designated search engine.

That slice of on-screen real estate has the potential to be enormously valuable, and Microsoft is the landlord. Internet Explorer 7 is the first Microsoft browser to have a built-in search box, while other browsers like Firefox, Opera and Safari have had them for some time. Google estimates that the boxes, when available, are the starting point for 30 to 50 percent of a user's searches, making them a crucial gateway to the lucrative and fast-growing market for advertisements that appear next to search results.

Microsoft has lost some ground in the browser market in the last year, mainly to Firefox, which is a Google ally. But Microsoft still holds more than 80 percent of the market. And Internet Explorer 7 is expected to be extremely popular because it is an improvement over Microsoft's previous browser, and because Microsoft will promote downloads of it and include it in Windows Vista.

That gives Microsoft the potential to use the browser to steer substantial traffic, and business, to MSN and away from rivals. MSN handled 11 percent of searches in the United States in March, down slightly from a year earlier, according to Nielsen/Net Ratings, a market research firm. That put it well behind Google, which had a 49 percent share, and Yahoo, with 22 percent.

Microsoft insists it has no intention of deploying its browser as a weapon in the search wars. But Google suspects otherwise.

In meetings beginning last year, Google told Microsoft of its objections to the company's plans to set MSN as the default search engine in Internet Explorer 7, according to Ms. Mayer of Google. Yahoo raised similar objections in a meeting with Microsoft last year, according to a Yahoo employee who was briefed on the conversation. Yahoo declined to comment last week beyond a statement: "We would be concerned about any company's attempts to limit user choice or change user preferences without their knowledge, and believe others would share that concern."