Microsoft has agreed to modify its Windows Vista operating system in response to a complaint filed by Google.Under an agreement with the Justice Department and 17 state attorneys general and the District of Columbia, Microsoft will build into Vista an option to let users select a search program on personal computers running Windows.The function, known as "Instant Search," allows Windows users to enter a search query and get a list of results from their hard drive that contain the search term.The agreement was made public as part of a joint report that the Justice Department and Microsoft filed late on Tuesday with the court overseeing Microsoft's compliance with a 2002 antitrust consent decree.The changes will be introduced in a service pack, or updated version of Windows Vista software. Microsoft said it anticipates a test version of the Vista Service Pack 1 to be ready by the year-end.Under the agreement, Microsoft also promised to provide additional technical information to third-party developers, such as Google, in order to optimise the performance of their desktop search service on Vista.