Kenneth Insana depends on Microsoft Corp.'s Windows operating system to make his computer run, but Yahoo and Google have become the guiding forces on his desktop.

Yahoo has been Insana's online home page for the past two years, largely because the Web site serves as a one-stop source of information, e-mail, entertainment, shopping and social networking.

When he can't locate what he wants with Yahoo, Insana heads over to Google.

"If I can't find something there, I figure I'm not going to find it anywhere," said Insana, a 29-year-old entrepreneur in Bellerose, N.Y.

Millions of other computer users are following a similar path on the Internet, threatening Microsoft's stranglehold on computer desktops for the first time since Netscape Communications pioneered the Web browser in the mid-1990s.