Microsoft is unlikely to have been surprised by Google's announcement late Tuesday that it is working on a new OS for people who "spend most of their time on the Web," as Google described it. From what has been revealed about the Google Chrome OS -- which won't be available until the second half of next year -- it aims to bring the ease of use that Google has brought to Web-based applications, such as search and chat, to netbooks and eventually to full-size PCs.

A Google OS is "something Microsoft has been worried about for a long time," said Matt Rosoff, analyst with Directions on Microsoft. He called it "the first significant threat to Windows in a very long time," although he said the threat may not become fully evident for another 10 years.

Still, Google's plan to exploit the popularity of low-footprint, low-cost netbooks could accelerate the need for Microsoft to reinvent the bulky, PC-centric version of Windows for consumers and businesses, as more people begin using applications that live on the Web rather than on their local hard drive.

It could also eventually force the company to develop one core version of Windows that can be used on any device -- be it a smartphone, netbook or PC -- similar to the way Apple moved downstream by adapting its Mac OS X software for use on the enormously successful iPhone, analysts said.

Microsoft has not been immune to the problem of marrying the increasingly Web-centric world with the desktop world, in which applications run on a thick client with a resource-heavy OS, but until now the company hasn't had to worry too much about it.

With Windows the de facto standard on PCs, people have been for the most part content to wait for Microsoft to deliver new versions of the OS, and connect to the Web and their favorite applications from there. And many business customers are tied to Windows by long-term contracts and application dependencies, which has kept them loyal to Windows for better or worse.

For its part, Microsoft has been working to hone its Web-based services and applications, and even removed some software from the forthcoming Windows 7 -- such as e-mail and photo-editing software -- in favor of Web-based versions that are more lightweight. Windows 7 will be available on PCs later this year.