One way to summarize Google’s week so far is to say that it picked fights with two of the biggest computing companies on the planet. On Monday it unveiled a bookstore that competes with Amazon’s Kindle line. Today it took the wraps off an attempt to displace Microsoft from the provider of the most popular operating system.

“With Chrome OS we have a viable third choice for OS on the desktop,” said CEO Eric Schmidt at the launch event in San Francisco, drawing a comparison with Windows and Mac OS. He spoke after a lengthy demonstration that showed off a new vision of the computer that strips away pretty much everything except the browser.

Fresh out of the box, a Chrome notebook boots up in a few seconds. After connecting it to the Internet, you log in with a Google account and are pitched into the Chrome browser. At that point, the setup is over. Chrome OS is little more than Chrome the browser.

In this new world you get all your applications through the Chrome webstore, an app store on the now familiar model (check this link later today to browse the Chrome Web store using the browser on an existing OS). You can browse, search, read reviews and click to install. Some are paid and some are free. Examples shown today included a version of EA’s puzzle game PopIt, a tablet-style NPR app that lets you drag and drop shows to build a custom playlist, and one from the New York Times that lets you choose alternative “skins” for the paper’s content.

Those apps can be powerful because of features built into Chrome that make it faster than any other browser, boasted Sundar Pichai, vice president of product management at Google. Many—including most games and the app from the New York Times—can also work offline because they are cached by the browser, a feature that will soon appear in Google’s online Microsoft Office competitor, Google Docs.

It adds up to an experience that looks compellingly simple. “Since 2004 it is very hard to name a new application outside the Web that has scaled to hundreds of millions of users,” said Pichai, “people live within their browsers on the Web but most of the code and complexity on their systems has nothing to do with the browser and the Web.” Chrome OS strips all that away, even updating itself without user input.

Very soon, certain users of the Chrome browser and fans of it on Facebook will get the chance to try that for themselves when they are invited to join a pilot program. They’ll receive a notebook commissioned by Google to test its new OS. Notebooks made by Acer and Samsung—built with Intel chips—will go on sale globally in mid 2011, although pricing hasn’t been announced. All of those devices will see battery lives in the region of eight hours while in active use, and weeks on standby.

It’s possible to overdose on simplicity, though, as evidenced by some things Chrome OS currently lacks. A “Cloud Print” service that will allow use of network-connected printers is not yet done, for example, and although the devices will have USB ports it is not yet possible to plug in, say, a camera and download photos.

Building Chrome was not just an exercise in subtracting from the conventional OS experience, though. It also introduces some technologies not seen before. One is a feature called verified boot, which sees the initial chunk of the OS installed on a part of the device that cannot be modified without physically taking the computer apart. “Every time you boot we use that safe part to cryptographically check every other part of the OS and we keep a known backup copy that we can revert to,” said Pichai. “We are very confident that it will be the most secure consumer OS shipped.”

Ultimately, though the core promise of Chrome OS is familiar: it’s an attempt to deliver on a suite of ideas and concepts about cloud computing and the Web that have been circling for years. Schmidt in particular has been here before. In the late 90s, in a previous life as CTO of Sun Microsystems, he pushed the “Network Computer,” a diskless notebook that relied on the cloud for everything. “Our instincts were right 20 years ago but our technology wasn’t mature,” he said today. “This time it does in fact work.”