Of all the changes in Windows XP, the biggest one is invisible to the eye. The guts of Windows XP aren't the same guts that were in Windows 9X/Me. Instead, Windows XP uses the underlying engine that is used in the Windows 2000 operating system. If you ignore IBM's old OS/2 Warp (and most people did), this marks the first time a corporate-grade OS has been made available to home users.
All previous Microsoft home operating systems have evolved from the old 16-bit MS-DOS operating system. The very first version of Windows wasn't much more than a pretty face—a graphical user interface, or GUI—grafted onto the underlying DOS command structure. Subsequent versions of Windows improved on the original, but still retained the original MS-DOS underpinnings. This was done to ensure compatibility with older applications and believe it or not, a fair number of DOS-based applications are in use today.