Windows was developed for IBM PC compatible computers (these were based on Intelx86 architecture), and to this day, almost all versions of Windows are made for this hardware-platform (although some versions of Windows NT were made for other architectures ). The popularity of Windows made IntelCPUs more popular and vice versa. In fact, the term Wintel became used to describe PC compatible computers running a version of Windows.
The term Windows is used as a collective term for several generations of products, which can be classified into the following categories:
16-bit Operating environments. Although they are often thought of as just graphical user interfaces or desktops, mostly because they use MS-DOS for filesystem services, 16-bit Windows versions already have their own executable file format and provide their own device drivers (graphics, printer, mouse, keyboard and sound). Most importantly, from the beginning they allow the user to (non-preemptively) multi-task graphical applications, something which competitors like GEM do not offer. Finally, they implement an elaborated segment-based software virtual memory scheme, which allows to run applications larger than available memory: code segments and resources are swapped in and thrown away when useless or memory becomes scarce and data segments move in memory when a given application has relinquished processor control. Examples include Windows 1.0 (1985) and Windows 2.0 (1987) and its close relative Windows/286.
Hybrid 16/32-bit operating environments. Windows/386 introduced a 32-bit protected modekernel and virtual machine monitor. For the duration of a Windows session, it provided a device virtualization for the disk controller, video card, keyboard, mouse, timer and interrupt controller. The user-visible consequence was that it became possible to preemptively multitask multiple MS-DOS environments in separate windows (graphical applications required switching the window to full screen mode). Windows applications were still multi-tasked cooperatively inside one of such real-mode environments. Windows 3.0 (1990) and Windows 3.1 (1992) perfected the design, notably thanks to virtual memory and loadable virtual device drivers (VxDs) which allowed them to share arbitrary devices between multitasked DOS windows. Most important, Windows applications could now run in 16-bit protected mode (when Windows was running in Standard or 386 Enhanced Mode), which gave them access to several megabytes of memory and removed the obligation to participate in the software virtual memory scheme. They still ran inside the same address space, where the segmented memory provided a degree of protection, and multi-tasked cooperatively. For Windows 3.0 Microsoft also rewrote critical operations from C into assembly, making this release faster and less memory-hungry than its predecessors.
Hybrid 16/32-bit operating system. With the introduction of 32-Bit File Access in Windows for Workgroups 3.11, Windows could finally stop relying on DOS for file management. Leveraging this, Windows 95 introduced Long File Names, reducing the 8.3 DOS to the role of a boot loader. MS-DOS was now bundled with Windows; this notably allowed to make it (partially) aware of long file names when its utilities were run from within Windows, but angered the competition. The most important novelty was however the possibility of running 32-bit multi-threaded preemptively multitasked graphical programs. There were three releases of Windows 95 (the first in 1995, then subsequent bug-fix versions in 1996 and 1997, only released to OEMs, which added extra features such as FAT32 support). Microsoft's next OS was Windows 98; there were two versions of this (the first in 1998 and the second, named "Windows 98 Second Edition", in 1999). This was an evolutionary enhancement, in much the same relation to Windows 95 as Windows 3.1 had been to 3.0. In 2000, Microsoft released Windows Me, which used the same core as Windows 98 but adopted the visual appearance of Windows 2000, as well as a new feature called system restore, allowing the user to set the computer's settings back to an earlier date. Compared to previous upgrades, comparatively few people bothered to switch to ME: by this time most power users had already jumped over to the NT family. This can be defended by the fact that Microsoft left little time for Windows Millennium to become popular before announcing their next version of Windows.
32-bit operating systems originally designed and marketed for higher-reliability business use with no DOS heritage. The first release was Windows NT 3.1 (1993, numbered "3.1" to match the Windows version and to 1-up OS/2 2.1, its main competitor at the time), which was followed by NT 3.5 (1994), NT 3.51 (1995), and NT 4.0 (1996); the latter of which introduced the Windows 95 interface. Microsoft then moved to combine their consumer and business operating systems. Their first attempt, Windows 2000, failed to meet their goals, and was released as a business system. The home consumer edition of Windows 2000, codenamed "Windows Neptune", ceased development and Microsoft released Windows ME in its place. Eventually "Neptune" was merged into their new project, Whistler, which later became Windows XP. XP finally rendered DOS obsolete, and since then a new business system, Windows Server 2003, has expanded the top end of the range, and the forthcoming Windows Longhorn will complete it. Windows CE, Microsoft's offering in the mobile and embedded markets, is also a true 32-bit operating system.
64-bit operating systems, the newest category, are designed for AMD's AMD64 CPU architecture, and Intel's Intel Architecture 64-bit, and EM64T. The 64-bit Windows family comprises Windows XP Itanium edition and x64 Edition and Windows Server 2003; Windows XP Professional and server x64 editions have reached RTM status in March 30, 2005. Itanium editions already came out in 2002. Early indications are that Windows Longhorn, the projected successor to Windows XP, will be released in both 32-Bit and 64-Bit versions.
A typical Windows XP desktop.
The most obvious feature of the more recent Windows versions (since Windows 95 and NT 4.0) is the desktop, which is similar to the "Workplace Shell" introduced by IBM for OS/2 2.0 in 1992, an object-oriented GUI running on the OS/2Presentation Manager. The Windows desktop has produced a significant change in the way people and computers interact; it is possible to perform many common tasks with very little computer knowledge, including some quite complex ones.
Windows XP introduced a new visual style dubbed "Luna", which updated the classic Windows style (a plain grey box look) with a more graphical appearance. The new style features bold colors and a larger titlebar and start button, leading many to call it the "playskool" interface (after a popular brand of children's toys)  and others to call it the "Teletubbies" interface (after the children TV show Teletubbies, also partly because of Windows XP's default wallpaper, a grassy plain with a blue sky, which resembles the set of the TV show) . Users can still elect to use the old Windows 95/2000 visual style.
There is a view that modern operating systems need to cater for the vastly increased user base with a lower average computer skill level and the increased power and complexity of modern computer systems. Therefore, some technically savvy users accuse the Windows interface of isolating the user from too much of the inner workings of the computer, making it more difficult to control and configure some system features, although many such features can be now controlled also from the command-line or by scripting. However, this has always been an issue to some extent with GUI operating systems, and, to a lesser extent, almost all operating systems, by definition.
Thought to be installed on over 90% of personal computers, Windows has achieved enormous market penetration due in large part to the domination of MS-DOS in the early days of PC compatiblecomputers (IBM-PC clones), and also because it is the primary platform for Microsoft Office and its proprietary document formats (which, however, are also supported by a large number of non-Microsoft products including the open-sourceOpenOffice.org office suite as well) used by most microcomputer users, as well as the fact that Microsoft is one of the few companies that license their OS to hardware manufacturers (for example, Apple does not license Mac OS X to anyone, nor does Sun license Solaris to anyone). Due to Microsoft's exclusive licensing agreements with many computer vendors, Windows comes pre-installed on most computers (as a bundled OEM version), making it the default choice for much of the market. Most consumers do not delete Windows and install another operating system. For some consumers, Windows is the only valid option as their computing environment is mandated by their workplace; for others, the wide variety of applications that is available for the platform limits their desire to switch to other operating systems. While many users have made an informed choice to run Windows, a large number are entirely unaware that there are other choices.
However this is set to change with some manufacturers such as Hewlett Packard offering laptops with Linux pre-installed. Some companies specialise in manufacturing computers with Linux pre-installed. A few years ago Dell offered computers with Linux pre-installed but randomly dropped the product soon after.
Security has been a major issue with Windows family products for many years. Most modern operating systems were designed for security in a multi-user and/or networked environment and have a relatively small number of security issues. Windows was originally designed for ease-of-use on a single-user PC without a network connection, and did not have security features built in from the outset. Combined with occasionally flawed code (such as buffer overruns) Windows has been the successful target of worms and virus writers numerous times. The Blaster worm of August 2003 is a recent example.
It is unclear whether the security problems with the Windows line are particularly systemic for Microsoft, or are so prevalent merely because Windows dominates the world's desktops, thus leading attackers to concentrate on Windows systems before any other type. Nevertheless, Microsoft publicly admitted their ongoing security problems shortly after the turn of the century and (according to their press statements) now claims to regard security as their number one priority.
Microsoft releases security patches through its Windows Update service approximately once a month, although critical updates are made available at shorter intervals. In Windows 2000 and Windows XP, updates can be automatically downloaded and installed if the user elects to do so.
Many security experts are advising Windows users to take steps to increase the security of their systems. A study conducted by Kevin Mitnick and marketing communications firm, Avantgarde, found that an unpatched Windows XP system only lasted 4 minutes on the Internet before it was compromised.  The AOL/National Cyber Security Alliance Online Safety Study of October 2004 determined that 80% of Windows users were infected by at least one spyware/adware product.  Much documentation is available describing how to increase the security of Microsoft Windows products. Typical suggestions include deploying Microsoft Windows behind a hardware firewall, running anti-virus and anti-spyware software, and installing patches as they become available through Windows Update. Some suggest replacing specific programs which are often targeted by crackers with alternatives, such as replacing the web browser Internet Explorer with Mozilla Firefox, and replacing the email client Microsoft Outlook with Mozilla Thunderbird.
In 2001, Microsoft started talking about a controversial security initiative called Palladium. The system has since been renamed "Next Generation Secure Computing Base". Palladium is a system intended to allow a program to verify that it is unmodified, and is running on "trusted" hardware with "trusted" drivers and a "trusted" architecture. This, in theory, assures the software that it is running on unmodified software and hardware. Palladium has a particular use in DRM systems. A side effect will be to lock out "untrusted" operating systems at the hardware level: this is likely to impact Linux in particular, since there is no single canonical Linux which can be certified, and many users consider the ability to customise the system one of its main attractions. Some conspiracy theorists hold that this is the secret reason behind the system, although in practice it is unlikely that Microsoft will try to prevent Linux vendors such as Red Hat from having their products certified. As of 2004, Palladium has not resulted in any commercial products.
Microsoft has a number of new initiatives planned or in progress: .NET, Next-Generation Secure Computing Base, and the "Longhorn" operating system, which is due in 2006 according to some Microsoft executives. Further down the road, there is the "Blackcomb" operating system, which is due some time around 2010, and will have both client and server versions.
Captive NTFS - An open-source effort to create a "wrapper" around the original Windows NTFS file-system DLL's in order to more fully implement compatibility between file read/write operations for Unix-like operating systems
ReactOS Project to make a compatible Open Source version of Windows