Windows NT 4.0 (1996)
Microsoft updated Windows NT to version 4.0, which brought several enhancements. Windows NT 4.0 inherited its Start bar and GUI from Windows 95, which made the two OSes look similar. They were functionally different, however, and Windows NT 4.0 was much more server- and business-oriented.
Windows 95 was also much more user friendly than Windows NT 4.0; the latter did'nt include features such as Plug and Play (PnP) and Device Manager. Microsoft also used a new hardware abstraction layer on Windows NT 4.0 to reduce the likelihood of a system crash, but that also made installing new hardware more complicated. Further, just like Windows NT 3.1, Windows NT 4.0 had rather high system requirements. The OS could technically run with just 16MB of relatively expensive protected memory, but Microsoft recommended at least 32MB of RAM. It also required at least 110MB of hard drive space.
Although Windows NT 4.0 was outshined by Windows 95, it was fairly popular for businesses and servers. It was far more stable than Windows 95, and it was possible for the OS to run for several days or even weeks without crashing. Anyone who lived through the 1990s knows the same cannot be said for Windows 95.
Windows 98 (1998)
Windows 98 was essentially a polished version of Windows 95. It included several new applications such as Outlook Express, Microsoft Chat, NetMeeting, and NetShow Player. The OS also included FrontPage Express and a Personal Web Server with a built-in Web Publishing Wizard that allowed you to develop, publish, and host websites on your home PC. The UI introduced in Windows 95 was integrated with Internet Explorer and given several new features, as well. Forward and back buttons were added to ease file navigation, and Microsoft made it easier to minimize and launch new windows.
Microsoft also wanted to make Windows 98 more stable than its predecessor. By implementing numerous tweaks to the kernel, system drivers, system registry, and system memory management, Microsoft was successful in making Windows 98 less prone to crashing. This also helped to improve overall performance.
Due to their user friendliness, affordability, and wide program support, Windows 95 and Windows 98 lead Microsoft to a place of dominance in the operating system market. Over the next few years, Microsoft further cement its dominance as numerous other companies went out of business.
Windows 98 SE (1999)
Around the year 2000, Microsoft released four operating systems in quick succession. The first of these was Windows 98 SE, which was essentially just an updated version of Windows 98 with numerous bug fixes. This version also included a few new programs such as Windows Media Player, and it shipped with an updated version of Internet Explorer and the new DirectX 6.1 API. Windows 98 SE is considered by many to be the best DOS-based OS created by Microsoft. Although it is no longer supported by Microsoft itself, fans of the OS continue to support it with unofficial service packs that improve the operating system's feature set and stability.
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Windows 2000 (1999)
Windows 2000 was released just a few months after Windows 98 SE and less than two weeks before the year 2000. (It was originally planned for release in 2000--hence the branding.) Essentially, the design goal for Windows 2000 was to add all of Windows 98's user-friendly features and programs to an OS based on Windows NT.
Windows 2000 received updated versions of most Windows 98 programs, and it shipped with several new features not found on any prior OS. It had unprecedented PnP support for USB and internal hardware devices. Like Windows NT 4.0, Windows 2000 was also rather stable. Microsoft also introduced Windows File Protection with Windows 2000, as well as enhanced software debug tools. This helped to improve system security and further improve overall stability.
Although Windows 2000 had a client-oriented version complete with games and multimedia features, it was mostly targeted as a server solution. The OS required just 32MB of RAM and 1GB of free storage space, but Microsoft recommended that users have 128MB of RAM and a 5GB HDD. Unlike Windows NT 4.0, Windows 2000 required at least an Intel Pentium 133MHz x86 CPU. With these easy-to-meet hardware requirements, Windows 2000 could cover all but the low-end home market, which was handled by Windows 98 SE.
This would be the last time Microsoft would use a general-purpose OS to target both servers and client workstation machines.
Windows Millennium Edition (2000)
Windows Millennium Edition (ME) was Microsoft's last DOS-based operating system and a direct successor to Windows 98 SE. To create Windows ME, Microsoft tweaked the Windows 98 SE kernel to limit software access to real-mode (DOS) in order to improve system stability and security, and to accelerate load times.
The UI was also updated to look similar to Windows 2000, and PnP and some other eatures were updated to have parity with Windows 2000. Ultimately, Windows ME proved to be a failure, and it may well be the least popular OS ever developed by Microsoft.
A key reason for Windows ME's failure in the market was its restricted real-mode access. Even in the year 2000, DOS-based software was heavily used; when Microsoft restricted real-mode access, it essentially removed backwards compatibility with a large number of DOS programs. Because of this issue, most people opted to either continue to use Windows 98 SE or switch to the more expensive Windows 2000.
Windows XP (2001)
Windows XP was released in August 2001, and it was the fourth OS released by Microsoft in a relatively short two-year period. Microsoft developed Windows XP as both a consumer- and business-oriented solution that could handle any non-server aspect of the market. The OS was in many was an upgraded version of Windows 2000 with enhancements to essentially all aspects of the software, except the server utilities, which were not included with Windows XP.
It would take several pages to adequately cover all of the new features and improvements in Windows XP, but suffice it to say that the OS was more user-friendly and stable than its predecessors. It also further simplified the process of adding new hardware to a system.
Windows XP is arguably the most popular operating system ever produced. By the time Microsoft discontinued Windows XP, more than one billion copies had been sold. Later Microsoft operating systems struggled to pull users away from Windows XP, and it's actually still quite common to see the OS in use today, 15 years after its initial release.
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Windows XP 64-bit Edition (2002)
Although Windows XP was primarily a 32-bit OS, Microsoft did develop 64-bit versions of the operating system. Windows XP 64-bit Edition was released in 2002, but it supported only Intel Itanium 64-bit processors. This version of Windows XP had a relatively short life due to its limited CPU support, and it was discontinued in 2005.
Windows XP Professional x64 Edition (2005)
Not long after Microsoft discontinued Windows XP 64-bit Edition, the company launched Windows XP Professional x64 Edition. Although the two OSes have similar names, they support completely different types of processors. Whereas Windows XP 64-bit Edition was designed for Itanium processors, Windows XP Professional x64 Edition supports x86-64 CPUs from both AMD and Intel. This eased adoption of 64-bit hardware and software, as the systems were capable of executing essentially any 32-bit x86 software.
Windows Vista (2006)
Windows Vista was designed as a successor to Windows XP, but it was ultimately unsuccessful in luring large numbers of users away from the aging OS.
Vista was designed with an updated UI and a wide number of other improvements. The most critical update to Windows Vista was its increased software security. Ironically, the increased security safeguards are in part what lead to Vista's downfall. Software was run in a non-administrator protected mode by default, which caused some programs to malfunction. When you attempted to change system settings or access more advanced system controls such as those found in Control Panel, Vista required you to verify (several times) that you wanted to continue as an administrator and alter the settings.
The idea was that the verification prevented users from accidentally giving malicious software access to the rest of the system, but it also made it more difficult for experienced users to adjust settings and fix issues.
Vista is also criticized for being overall slower than Windows XP due in part to the aforementioned security improvements.
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Windows 7 (2009)
Windows 7 was designed essentially as a heavily polished version of Windows Vista. Although Microsoft continued to push for increased security, the company also listened to feedback about Windows Vista and made Windows 7 less intrusive. You were still required to tell software to run in Administrator mode, but far less often. Performance was also significantly increased over Windows Vista and Windows XP. This performance gap is the most pronounced when you examine compute-intensive workloads that take advantage of new processor instructions found in Windows 7, such as Advanced Vector Extensions (AVX). Windows 7 was also more stable than its predecessors, which encouraged users to switch from the somewhat crash-prone Windows XP.
Windows 7 is regarded as Microsoft's second most successful OS of all time, behind XP. Although the OS struggled to pull users from Windows XP, it has had a steadily growing user base since its introduction and is widely used today.