At first it was a graphical shell designed by Microsoft that added much-needed 32-bit preemptive multitasking to the otherwise inferior monolithic monotasking 16-bit DOS kernel. These shells are the "win9x" series of Windows--every version from Windows 95 through Windows ME (previous versions of Windows were 16-bit). A parallel project called Windows NT (which would later evolve into Windows 2000, Windows XP, and soon into Windows Vista), a true 32-bit preemptive multitasking OS (read: no DOS), is in the process of replacing the win9x series. All are surprisingly insecure.
Architecturally, these are all inferior OS's when compared to their main competition: UNIX and its derivitives (i.e. Linux and *NIX OS's, and Mac OS X). Arguably, they only managed to gain a marketshare because the UNIX community had been in a state of decline in the mid 1990s, and Linux wasn't user-friendly enough to take the market (i.e. there was a power vacuum). All Windows OS's and shells are plagued with numerous security holes inherent to their monolithic design: because all essential parts of the OS's are so tightly integrated, not only is it extremely difficult for a development team to attempt to fix a problem without creating more problems due to the interdependencies inherent to the monolithic structure, but also any security flaws in ANY component of the OS (or shell) could be used to somewhat easily exploit any other system components. This, coupled with the fact that none of the Windows OS's are true multi-user systems (unlike time-sharing systems like UNIX and its derivitives), thus making it easier for a user to do significant damage to the system without using the administer account, makes all of the OS's undesirable for mission-critical applications (including server use), or even for regular internet use. In fact, security analyses show that Windows suffers from so much malware and cracker/script-kiddie attacks mostly for its flawed design, rather than its popularity. One should also note that popularity is only indicative of effective marketing, not quality.
The only "worthy" use for this software is to play games--its large userbase has attracted 3rd-party multimedia programming and hardware development firms more than any other OS in history. Consequently, most hardware in the computing world works or can be made to work with Windows. Even now, this unique feature is deteriorating as more and more developers cross-compile their software for Mac OS and Linux (i.e. NVIDIA Corporation writes a universal driver for their video hardware that will work with Windows, Mac OS, and Linux).
Even with a broadband connection, an unpatched installation of Windows XP usually cannot download and install the necessary updates before it is infected or exploited.
Mac OS X, Linux, BeOS, Mac OS Classic, and other (somewhat) current OS's with merit, however, are either virtually immune to malware (UNIX and derivitives are compartamentalized and modular, making it virtually impossible for a user, or a program executed by a user, to take control of a system without root privilages), or are too obscure to effectively develop software for in the first place.