By STEPHEN LABATON

WASHINGTON, June 9 \ufffd Nearly a decade after the government began its landmark effort to break up Microsoft, the Bush administration has sharply changed course by repeatedly defending the company both in the United States and abroad against accusations of anticompetitive conduct, including the recent rejection of a complaint by Google.

The retrenchment reflects a substantially different view of antitrust policy, as well as a recognition of major changes in the marketplace. The battlefront among technology companies has shifted from computer desktop software, a category that Microsoft dominates, to Internet search and Web-based software programs that allow users to bypass products made by Microsoft, the world\ufffds largest software maker.

In the most striking recent example of the policy shift, the top antitrust official at the Justice Department last month urged state prosecutors to reject a confidential antitrust complaint filed by Google that is tied to a consent decree that monitors Microsoft\ufffds behavior. Google has accused Microsoft of designing its latest operating system, Vista, to discourage the use of Google\ufffds desktop search program, lawyers involved in the case said.

The official, Thomas O. Barnett, an assistant attorney general, had until 2004 been a top antitrust partner at the law firm that has represented Microsoft in several antitrust disputes. At the firm, Justice Department officials said, he never worked on Microsoft matters. Still, for more than a year after arriving at the department, he removed himself from the case because of conflict of interest issues. Ethics lawyers ultimately cleared his involvement.

Mr. Barnett\ufffds memo dismissing Google\ufffds claims, sent to state attorneys general around the nation, alarmed many of them, they and other lawyers from five states said. Some state officials said they believed that Google\ufffds complaint had merit. They also said that they could not recall receiving a request by any head of the Justice Department\ufffds antitrust division to drop any inquiry.

Mr. Barnett\ufffds memo appears to have backfired, state officials said. Prosecutors from several states said they intended to pursue the Google accusations with or without the federal government. In response, federal prosecutors are now discussing with the states whether the Justice Department will join them in pursuing the Google complaint.

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Google has asked the court overseeing the antitrust decree to order Microsoft to redesign Vista to enable users to turn off its built-in desktop search program so that competing programs could function better, officials said.

State officials said that Mr. Barnett\ufffds memo rejected the Google complaint, repeating legal arguments made by Microsoft.

Before he joined the Justice Department in 2004, Mr. Barnett had been vice chairman of the antitrust department at Covington & Burling. It represented Microsoft in the antitrust case and continues to represent the company.

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Prosecutors from several states said that they believed that Google\ufffds complaint about anticompetitive conduct resembled the complaint raised by Netscape, a company that popularized the Web browser, that was the basis of the 1998 lawsuit.

Richard Blumenthal, the Connecticut attorney general, declined to talk about the substance of the complaint, or which company made it. But he said the memo from Mr. Barnett surprised him.

\ufffdEyebrows were raised by this letter in our group, as much by the substance and tone as by the past relationship the author had had with Microsoft,\ufffd said Mr. Blumenthal, one of the few state prosecutors who has been involved in the case since its outset.

\ufffdIn concept, if not directly word for word, it is the Microsoft-Netscape situation,\ufffd Mr. Blumenthal said. \ufffdThe question is whether we\ufffdre seeing d\ufffdj\ufffd vu all over again.\ufffd

The administration has supported Microsoft in other antitrust skirmishes as well.

Last year, for instance, the United States delegation to the European Union complained to European regulators that Microsoft had been denied access to evidence it needed to defend itself in an investigation there into possible anticompetitive conduct. The United States delegation is led by Ambassador C. Boyden Gray, who had worked for Microsoft as a lawyer and lobbyist.

Robert Gianfrancesco, a spokesman for the delegation, said that Ambassador Gray had not formally removed himself from involvement in Microsoft issues but was not involved in the complaint to European regulators, which was handled by other American diplomats in the delegation.

In December 2005, the Justice Department sharply criticized the Korean Fair Trade Commission after that agency ordered major changes in Microsoft\ufffds marketing practices in Korea.

And in 2004, the Justice Department criticized the European Commission for punishing Microsoft for including its video and audio player with its operating system.

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