LOS ANGELES -- Microsoft is expected to make a "major" announcement after the market closes Monday.

Speculation about the Los Angeles media event has ranged from an unveiling of a new tablet computer that uses low-power chips to a new system that will use an upcoming version of Windows to help people access TV shows and movies across a range of devices.

Microsoft announced a $300 million, 17.6 percent stake in Barnes & Noble's Nook e-book business in April, so a tablet-focused update is also expected.

The Windows 8 system is expected to be released in September or October, in time for devices that will ship for the holiday season. Windows 8 is considered to be the biggest change in decades to the widely used operating system from Microsoft Corp. It has been designed so that it can run desktop, laptop and tablet computers, giving PC makers a better chance of competing with Apple Inc.'s iPad.

"This will be a major Microsoft announcement – you will not want to miss it," the company said in an email invite to the media on Thursday. A company spokesman declined to comment further.

The event, set to start at 3:30 p.m. Pacific time, is being kept under such tight wraps that the company has declined to provide its location until Monday.

Also on HuffPost:

Check out the slideshow to see some of the biggest features of Windows 8.
Loading Slideshow...
  • Touchscreen Password To Unlock

    The Microsoft team showed off a neat new password system to unlock Windows 8 computers. When your computer is locked, normally you have to type in a text password to regain access to the system; Windows 8 gives the user an option to unlock via a combination of touches and swipes, which might sound familiar to Android users.<br> <br> When setting up your password, you choose a picture, and you select where on the picture you want to tap and swipe in order to unlock the screen. For example, at the Windows Build conference, Windows Corporate Vice President Julie Larson-Green showed off her picture-password: a photo of her daughter (above) standing on a pier holding a glass of lemonade appeared, and Larson-Green tapped on her daughter's nose, then on the glass of lemonade, then drew a line from the edge of the pier to the edge of the glass of lemonade. Voila! The screen was unlocked. <br>

  • Email Notifications On Lock Screen

    Speaking of that screen, Windows 8 has taken another cue from mobile operating systems and will automatically show on the lock screen relevant information like upcoming calendar events and emails received when the user was away from the keyboard. This is a small upgrade, but it eliminates the need to unlock your computer just to see if you missed anything while you were gone: Windows 8 will tell you from the moment you return your screen what happened during your time away. <br> <br> The lock screen also displays battery information, time and any instant messages you may have missed while you were away from your device.<br> <br> You might be thinking to yourself (à la George Bluth in the "Spring Breakout" episode of Arrested Development), "What does that save, like two seconds?" But, if you've used a Windows or Android smartphone with this feature, you know how helpful it can <em>feel</em> to know immediately about what you missed when you were unplugged.

  • Really Fast Booting

    <a href="http://www.huffingtonpost.com/2011/09/13/windows-8-pictures_n_960301.html" target="_hplink">This one was teased ahead of the Build Conference</a>, but they showed it off on-stage anyway, and it is still very impressive. Windows 8 will apparently boot in 8 seconds, thanks to a new process of putting the kernel session to sleep rather than closing it out altogether and having to reboot it completely. See the video (above) for a demonstration of how fast a computer running Windows 8 will be able to boot up from no power to start screen.

  • System Refresh Without Deleting Music, Movies Or Documents

    No, this doesn't mean you can throw away your external hard drive or cancel your Dropbox account. But it may prove to be a convenient and hardware-less way to restore your system without losing all of your precious, precious media, or that novel you've been working on. A push of the button from the Control Panel activates the system refresh, making it pretty convenient for the average PC user.

  • The New Task Manager

    As Sinofsky noted at the Build Conference, it's been two decades since Microsoft redesigned its task manager, and Windows 8 brings a new look and interface to what is (unfortunately) one of my most used Windows utilities. <br> <br> The new task manager looks great--easy to read and use, with all the pertinent information lined up in columns. As a bonus, the task manager also allows you to add and delete which programs you want to automatically load at startup (hallelujah!).<br> <br> Those "suspended" apps you see, by the way? When you are running apps in the tablet-ified 'Metro' view, they stop running when you switch over to 'Desktop' view, saving you CPU usage. Good idea, Microsoft.

  • Split Keyboard For Thumb-Typers

    Are you a thumb-typer? When you hold a tablet, do you type with your thumbs rather than all of your fingers? Or, do you type a lot on the go, where you can't put your slate down for proper Mavis Beacon typing technique?<br> <br> Then the thumb-typing keyboard--selectable from the keyboard menu--might just be for you.

  • The Share Charm

    Sitting on the start bar in 'Metro View' is the "Share Charm," a little button that pulls up a sidebar (seen above) for easy sharing through a number of different apps. Microsoft put much of its focus on interactivity and connectivity--from apps playing well with each other, to the fact that all Windows 7 programs will run on Windows 8, to putting much of Windows Live in the cloud--and the Share charm is no different. It's a handy little utility baked into Windows 8 that allows users to share what they're looking at with anyone in their address book using the automatic Share program. Select what you want to share and who you want to share it with, add an optional message and press 'Send."

  • More Free RAM

    If all of these features, multiple interfaces and visually-striking touch-and-slide systems look like they use a lot of RAM--well, they don't. According to Sinofsky, Windows 8 takes up 281MB to run on startup, versus 404MB for Windows 7 (and this is the Windows 8 Developer's Beta!). Another encouraging sign from an operating system that has historically been accused of memory hogging. <br> <br>