Microsoft is about to deliver an update to Windows 8.1, its second significant set of changes since the launch of Windows 8 less than 18 months ago.

Last week, someone in Redmond inadvertently left the final update packages available on Windows Update for anyone to install. I’ve had a couple days to use the newly updated Windows interface on a handful of machines.

You can see exactly what’s changed in the gallery that accompanies this post: "A close-up look at the Windows 8.1 Update." Here's some background on why this update exists.

The biggest mistake Microsoft made with Windows 8 was to deliberately eliminate some of the touchstones of the Windows 7 desktop interface , while still leaving most of that desktop intact. As I wrote a few months after the original release of Windows 8:

That decision alienated many desktop users and created a wedge issue that has distracted from the many impressive accomplishments in Windows 8. I know some people (myself included) who have adapted to the new ways and even prefer them. Those who would rather stick with the old paradigms can't catch a break from Microsoft, though. They need to tweak the system extensively and use third-party utilities to achieve the desired result.

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Windows 8.1 was a first, very large step on the road to rectifying that mistake. This update—let’s call it Windows 8.1.1—is a continuation of those changes, designed to make the new OS work more smoothly on conventional PCs driven by a keyboard and mouse.

A word of warning: If you’re a Metro hater, this update will do almost nothing to make you feel all warm and fuzzy. The Windows 7-style Start menu does not make a comeback here (although Microsoft has said it intends to include a Start menu in a future update). This update sticks with the fundamental design principles of the Windows 8 interface, and nothing in it comes close to restoring the Windows 7 desktop interface. It is still the Windows 8.x interface, evolved, with that evolution clearly driven by powerful negative feedback.

Some of the changes that are at the core of the Windows 8.1 Update won’t really be visible until we see new PCs with this version of Windows installed as the base operating system. For traditional PCs that are designed for use with a keyboard and a mouse, the new OS will be configured to go to the desktop by default. It will also be less of a disk-space and memory hog, making it possible for this version of Windows to run on tablets with as little as 16 GB of flash storage.

If you install this update when it arrives in April (via Windows Update, not via the Windows Store), the first thing you’ll notice is a series of changes to the Start screen. Alongside the picture and name of the current logged-in user are a new Power button and a Search button. Both of them are designed to reduce the need to play Where’s Windows? with the well-hidden Windows 8 Charms menu.

There’s also an option to right-click on tiles on the Start screen or in the more complete Apps view screen, which lists every installed desktop program and app. That option exposes a new, familiar-looking context-sensitive menu so you can resize those tiles or pin them to the Start screen or to the taskbar.

Oh yeah, that’s the really big change in this update: The taskbar is no longer just for the desktop. It can include Windows Store (Metro style) apps, so you can switch between desktop programs and Windows Store apps with one click. And the new, unified taskbar is available from the Start screen and from any Windows Store app, with a simple gesture.

Move the mouse to the bottom of the screen in Windows 8.1.1 and the taskbar appears. I’ve seen some complaints that the action is inconsistent. It works fine on multiple test systems here. I think what others are seeing is a deliberate design decision. If you move the mouse pointer casually to the bottom of the screen, the taskbar doesn’t pop up as a distracting element. If you really want to see the taskbar, you move the mouse with purpose, or move it to the bottom of the screen and then, after a very brief pause, move it down ever so slightly.

From the taskbar, you can preview any running program or Windows Store app, just as you can on the desktop, and switch to it with a click.

The other big complaint from early users of Windows 8 was that it was impossible to figure out how to close or switch away from Windows Store apps. The solution in the Windows 8.1 Update is a new title bar, with Minimize and Close buttons, which appears when you bump the mouse up against the top of the screen.

What’s noteworthy about all these changes is that they appear only when you use a mouse. If you use a touchscreen to navigate through Windows 8, you’ll see the taskbar only on the desktop, and you’ll only see the new title bars if you use a mouse.

There are a few additional usability tweaks in this update, including some welcome changes in Internet Explorer. There are also fixes you can’t see, aimed at improving security and performance and swatting bugs.

Will this update quiet the I-hate-Metro crowd? Probably not. Will it make Windows users on desktop machines more productive? Almost certainly.