2001: Windows XP
Windows XP, released in August 2001, was a breakthrough in several respects. It was the first version of Windows that did not use DOS as part of its underlying architecture, and the first to be offered in both 64-bit and 32-bit editions. XP combined the desktop version of the secure and stable enterprise-oriented Windows NT/2000 line with the consumer-focused Windows line. (The Windows Server OS line has continued separately from the desktop line.)
It was far more stable than previous versions of Windows and featured a significantly revamped interface that was brighter, more colorful and more contemporary-looking. Drop shadows were added to icon labels, windows were given a more rounded look and visual effects such as fading and sliding menus were added. Windows XP introduced a slew of new features, including background themes and remote desktop, which allows a PC to be controlled remotely via the Internet or a network.
Windows XP shipped in multiple versions, most notably Windows XP Home Edition and Windows XP Professional. Even though it was introduced nine years ago, XP remains the most-used version of Windows, and it's still available as a downgrade option on new PCs that run the Windows 7 Professional or Ultimate editions. Windows XP requires a Pentium 233-MHz processor or the equivalent (a 300-MHz model is recommended), at least 64MB of RAM (128MB is recommended) and at least 1.5GB of available space on the hard disk.