Google Inc. released free software yesterday that lets people simultaneously search the Web and their personal computers for information, a move analysts described as a potential blow to rivals Microsoft Corp. and Yahoo Inc. in the race to woo more searchers.

Google Desktop Search offers what Microsoft has been trying to develop for more than a year -- the ability to let people enter one search term and see files relevant to that topic from both their computers and the Web displayed together.

"This gives Google a huge first-mover advantage in desktop search," said Charlene Li, principal search analyst for Forrester Research, a market research firm. She predicted the software would be especially popular with heavy computer users, who store many files on their machines and need help sifting through them.

"It's ironic that until now, it's been easier to search 6 billion documents on the Internet than it has been to find a single file on your hard drive," Li said.

Google's new software, available as a download from http://desktop.google.com, not only indexes the full text of e-mail messages and word processing documents, it also gives people the option of creating a searchable archive of all Web pages they visit and all instant messages they send and receive with America Online Inc. software.

"The goal for the application was for it to behave like a photographic memory for your computer," said Marissa Mayer, Google's director of consumer Web products. "So in addition to being able to search all of the files on your computer, it also indexes the Web pages you have seen."

Some privacy advocates raised concerns about the new software, since the product indexes everything from online communications to files stored on a personal computer. Google said no documents from any user's computer would be sent to the company, stored on its computers or saved anywhere on the Web. And the company noted that the software gives users the ability to block it from recording online chats and visits to Web sites, and separately provides a way for people to turn it off for 15-minute intervals.

But Gary Price, a search specialist who runs a Web reference site called ResourceShelf.com, said the new archiving capability could raise privacy issues by making it easier for someone to sit down at a person's computer and snoop through his hard drive. That could be troubling in the workplace, he said: "In a couple of minutes, people can search your entire computer and find anything in any one of your documents."

David M. Burns, chief executive of Copernic Technologies Inc., which recently released its own free desktop search product, said his company spurned the idea of combining Web and computer searches for privacy reasons. When people are looking for private material, they may unwittingly choose the unified search option, he said, which will send their keyword over the Internet to run a Web search at Google.

"I don't think people will like having their private keyword sent over the public Internet," Burns said.

The new Google software is the company's first major innovation since its initial public offering in August, when it sold shares to investors for $85 each. Yesterday, Google stock closed at $142, up $1.10.

Analysts who tested the software say it is simple and fast, partly because it operates the same way Google does on the World Wide Web, by creating an index of the files it finds in advance and then searching that index when someone enters a query. That makes it speedier than the approach used by the search software built into Windows.

Google's new product is "very, very good," said Danny Sullivan, editor of SearchEngineWatch.com, an online newsletter that tracks the search engine industry. Sullivan said one of its most useful features is the way that it stored a copy of all the pages he visited online recently and then made that personal Web surfing history available to him.

"It improves your Web searching," Sullivan said. "This leaves me feeling that integrated search really is useful."

In the past month, both Yahoo and Ask Jeeves Inc. have released personalized search services that let users create archives of their Web surfing histories. But unlike Google's new software, those approaches store the files online, rather than on a user's computer, as Google does, and do not include desktop search. Dulles-based America Online is testing a new technology for desktop search that it anticipates releasing next month, a company official confirmed.

Google's release of the software ahead of Microsoft and the others may pose a problem for competitors, Sullivan said, because the personal archiving capability likely will grow more valuable to users over time, making it harder to switch to another search engine product. But Jim Lanzone, vice president of products for Ask Jeeves, said Google's head start gives it a limited edge. Ask Jeeves, for instance, plans to release its own offering before the end of the year. "Desktop search is in its very early stages," he said. "There is not an immediate mass market for this."

While Microsoft has been promising to develop a new desktop search product, the company has pushed back its timetable. "Our focus is on helping consumers get faster, cleaner and easier access to the information they want, not on what other companies are doing," said Justin Osmer, product manager for Microsoft's MSN division. "We plan to offer desktop search with updates to our existing service within the next year."

Microsoft does have software to search e-mails but has not unveiled a way for users to simultaneously search computer files and the Internet. "This is a big challenge," Osmer said.

Yahoo said it also is exploring desktop search software. "Yahoo remains highly focused on evolving our products to empower users to manage all their digital content wherever it may reside -- the Web, desktop or Yahoo," said spokeswoman Stephanie Ichinose.

Google's product works only on computers running Windows XP or 2000. It indexes the full text of certain documents, including those created in Microsoft's Outlook and Outlook Express e-mail programs, Microsoft Word, Excel and PowerPoint. With other files, such as photographs and music, it simply indexes the file name.

People who install the desktop software can run Google searches a number of different ways -- looking for files stored only on their computers, for files stored only on the Internet or for a mixture of both.

Google, which profits by showing ads related to Web queries, does not plan to place ads beside search results from personal computers. However, Google will show ads beside Web results when users choose to see results from both the Internet and their computers.

Mayer said early testing shows that most people will use the new software to search the Internet and their personal computers simultaneously, which will add to the number of Internet searches done through Google. "As a result, we will serve more Web results pages and more ads, and those ads have more chances of getting clicked on. So there will be incremental Web search revenue from this product," Mayer said.

Once the Google search technology is installed on a personal computer, it will transmit basic data daily to the company about usage patterns. For example, it will tell the company how often Google is being used to search personal computers, how often it is used to search the Web, and how often simultaneous searches are done.

Mayer said that daily feed will not, however, transmit any personal information to Google, adding that it is typical for major software programs to capture similar data. Users can also opt out of sending some usage data back to Google, though not all of it.

"This is the most personal information Google has ever dealt with," Mayer said of the new desktop search technology. "We take user privacy and user trust very seriously. And we have throughout the entire development of this product."

Computers with the new Google Desktop Search are on display in New York at yesterday's debut.