Microsoft Corp. on Monday joined the battle for supremacy in so-called desktop search, introducing software for quickly locating files on personal computers that challenges Google's two-month-old rival product.

The free software — like Google's Desktop Search compatible only with the Windows XP and Windows 2000 operating systems — scours and indexes users' hard drives so information can quickly be located with a simple query.

Microsoft's MSN Toolbar Suite lets users search for keywords located in most common document files, from e-mails to instant messages to PDFs, just as easily as they might search the Internet.

Google's rival product does not support searches for keywords in PDF files but, unlike the Microsoft product, does record all Web pages viewed by the user.

Both products are so-called beta versions, meaning the company acknowledges there may be flaws.

Desktop-searching has become an incredibly competitive field. Yahoo last week announced plans to launch its version in January; Ask Jeeves plans to announce a service this week; and AOL is expected to offer desktop-searching early next year.

Yusuf Mehdi, Microsoft's corporate vice president in charge of its MSN online division, said Microsoft's service differs from Google's in important ways. First, it allows the searches to be performed in the program running at the time — for example, if a user conducts a search while using Microsoft Outlook, the company's e-mail and calendar program, the results will come back in Outlook.

Unlike Google, Microsoft's service isn't browser-based and doesn't include Web pages in its search results. Mehdi said that helps allow for more powerful desktop searches — including those of Outlook calendar or contacts files and others that Google doesn't. The Microsoft product can also search network drives, which Google's cannot.

Mehdi said Microsoft's tool provides greater privacy protection because it searches through information based on each user who logs in. If one person uses a computer for personal banking, the next person using that machine won't be able to access the sensitive data, he said.

Google has indicated it plans to add similar capabilities to its program.

Charlene Li, an analyst with Forrester Research, said Microsoft's service is more powerful than Google's, but may also be less intuitive. Someone looking for apple pie recipes would have to conduct separate searches to find recipes on the Internet in addition to recipes on his or her hard drive, she noted.

"Google is a little cleaner," she said. "There's a lot more control that Microsoft is giving, but it's going to take people some time to figure out how to use it."

Companies are pressing to offer desktop-searching capabilities as a way to retain users.