In 1993, I was an Amiga user, a proud Amiga user, really, and openly disdainful of Microsoft and its products. At the time, the current version of Windows was 3.1, and while this was the version of the OS that catapulted Windows ahead of the Mac for good, I viewed it at the time as something that just happened to suck slightly less than its predecessor. Most readers probably aren't familiar with the Amiga, but suffice to say that we had true multitasking almost a decade before the Mac and Windows, and it worked on floppy-based systems too.

The Amiga was ahead of its time.
Not that the Amiga was perfect. Its reliance on TV-friendly, low-resolution displays, specialized chipsets, and obscure hardware capabilities were all huge advantages in the early PC days, but as Apple, Microsoft and other companies surged forward, especially on the software side, and PC hardware (sort of) caught up, Commodore (the makers of Amiga) were left in the dust.
And in 1993, I had briefly returned to school in an effort to become a developer. Of course, back then, as now, higher education was all Mac and PC, and I quickly began examining the then-current versions of Windows (as well as IBM's ill-fated OS/2, of which I was a big but temporary fan). At the time, there were three: Windows, Windows for Workgroups, and Windows NT. The first was, of course, the mainstream desktop OS we all know and love, and a shell on top of a shaky MS-DOS architecture. WfW, meanwhile, was Microsoft's network-optimized small business offering, and it offered a few key advantages over its mainstream brother, most notably the 32-bit underpinnings that would later be added to Windows 95. And NT was, at the time, a non-starter: While it looked like Windows, it was big, heavy, and slow, and couldn't run effectively on the PCs of the day.
We all know the story about how Microsoft eventually merged its DOS- and NT-based products into a single cohesive platform, but it's worth noting that in 1993, that change was still 8 years away and little more than a pipedream at the time. But what these three different OSes did share, aside from an underlying programming model, was a common shell. It was called Program Manager, and while it seems quaint today, it was the UI on the most commonly-used OS of the early 1990s.

Windows 3.x
Program Manager was based on two earlier Microsoft UIs, the original Windows UI, which was first called Interface Manager, and PMShell from OS/2 (where "PM" does indeed stand for "program manager"). It was one of several "managers" in early Windows versions, the others being File Manager (for managing files, of course), Print Manager, and Task Manager. But Program Manager was the shell, a special application that served as the Windows user interface, or what we might today call the "user experience."
Program Manager and the other Windows manager applications were tossed aside in Windows 95, which to date is the last time that Microsoft substantially revolutionized the Windows user experience. So instead of separate manager programs, Windows 95 (and then, later, NT 4.0) provided a single, more cohesive UI, called Windows Explorer.

Microsoft's own explanation of the UI changes it made in Windows 95 have some bearing today. "The overreaching goal of the UI in Windows 95 is to make PCs even easier to use for all people," the company noted in its self-published book Introducing Windows 95. "Novices want learning how to perform a task to be easy. Experienced users want to do more with their PCs, and they want efficiency and flexibility." (Re-read that with the Windows 8 Start screen in mind.)
Windows 95 made tasks like launching apps, task switching, and finding files discoverable for the first time. And it did so via the then-new Explorer shell, which featured all of the common user interface elements we still use today, including the Start Menu, taskbar, tray notification area, and a desktop on which you could place files and shortcuts. (The Windows 3.x Program Manager did not support placing files on the desktop, though minimized application buttons would appear there.)
(If you're interested in the design goals behind the Windows 95 user interface, I also recommend Adrian King's Inside Windows 95.)
Since then, the Windows user experience has evolved pretty steadily with only sparks of revolution. Windows 98 offered web integration bits, most of which later disappeared or were deemphasized, including a web-based Active Desktop, Back and Forward buttons in the shell, and web-like single clicking of onscreen elements. A Quick Launch toolbar was added to the taskbar, providing a first move towards application launching outside of the Start Menu (or desktop). And Explorer windows started providing more graphical views, including image thumbnails.

Windows 98
Windows 2000 and Me, both released in 2000, offered only minor, evolutionary changes to the Windows user experience, with a web-like preview pane in Explorer windows and a new font treatment.

Windows Me

Windows 2000
Windows XP was a fairly radical looking user experience change until you realize that all the colors and new capabilities didn't really change the basic structure of the UI, which still consisted of a Start Menu, taskbar, tray, and desktop. But Windows XP did feature a dark, saturated default color scheme (blues and greens instead of the old gray), and a new-look and larger Start Menu with customization capabilities. So it was a jarring shift for the day, which we forget because XP remained in the market for so long.

Windows XP
Windows Vista, of course, ushered in the current era with its Aero glass take on the Windows Explorer UI. But beneath the see-through, glass-like windows and occasional "live icons," Vista's Explorer really just provided a prettier take on the same old desktop UI, again. So a big change, visually, but not that big of a change from a usability perspective.

Windows Vista
In Windows 7, which was otherwise just a minor update to Vista, Microsoft did make a curiously major change to the ways in which we find and launch applications: It added this functionality directly to the taskbar for the first time. So while we had been typically using the Start Menu for this activity since Windows 95 (and/or the Quick Launch toolbar since Windows 98), with Windows 7, Microsoft formally started moving to a model where the program shortcut icons were always visible, in a preview of sorts of the Windows 8 Start screen to come. In this way, Windows 7 is to Windows 8 as Windows for Workgroups 3.11 was to Windows 95, a stealthy preview of what was to come.

Windows 7
In a related move, Microsoft made web sites and apps available as taskbar- and Start Menu-based shortcuts starting with Internet Explorer 9. This can and should be seen, too, as an interim capability between Windows 7 and Windows 8 since Windows 8's Start Screen is, of course, based on Internet Explorer and its apps are basically just web apps. See how this is all coming together?
We don't know a lot about Windows 8 yet. But we do know this: It will feature a Start screen as its default user experience. That is, this new Start screen won't be an option, like Media Center or some other app that you launch on your own. It's the default shell, the screen that comes up when you boot Windows and logon. In this sense, the Start screen is replacing Windows Explorer as the shell. But from a user experience standpoint, the Start screen is really just replacing the Start Menu and the taskbar, which were previously used for finding, pinning, and launching applications. In Windows 8, we'll use the Start screen for those activities.

Windows 8 Start screen
The classic Windows desktop (along with it's Start Menu and taskbar) are still available, of course, in deprecated form, in the same way that Program Manager, the classic Start Menu, and other legacy UI bits were available in previous Windows versions, after they were replaced, and before they were retired for good. You can expect these desktop UI bits to disappear over time, as developers move the majority of apps to new app models (like the rumored Jupiter apps platform I first reported on in January). But Windows 8, like all Windows versions before it, will not throw out the proverbial baby with the bath water; Microsoft cares about backwards compatibility.
Also know that the Windows 8 Start screen borrows from UIs that aren't even related to the Windows shell, though most of them are at least cursorily related to Windows itself. These include UIs from Media Center (several versions), Ultramobile PCs, Portable Media Center, Zune PC software (several versions), Zune HD, and of course Windows Phone. Go ahead, admit it: You forgot about half of those. But this stuff represents years and years of experience, so don't believe for a second that the Windows 8 Start screen is somehow untested or unproven.
However you feel about the Start screen, at least understand this: As you follow the progression of the Windows user interface, especially from Windows 95 to today, you can see that the Start screen is both an evolutionary update to previous app launching methods (Start Menu + taskbar in Windows 7) and a revolutionary update to the way Windows looks and works (i.e. the "user experience"). Whether it's successful or not is in some ways immaterial to this discussion: The important bit is that the Start screen is, in retrospect, an obvious next step given the changes of the past and the current market conditions. And I welcome any opportunity for Microsoft to stretch its creative muscle and work some revolution into that evolution it loves so much.