Which version of Windows was released in 2001?

The version of Windows released in 2001 was Windows XP.

Microsoft released Windows XP to manufacturing on August 24, 2001, followed by general retail availability on October 25, 2001. The name “XP” refers to “experience,” reflecting Microsoft's effort to create a unified Windows system for both home and professional users.

Windows XP was built on the Windows NT kernel rather than the older Windows 95 and MS-DOS foundation used by consumer editions such as Windows Me. Microsoft therefore positioned Windows XP Home Edition for home computers and Windows XP Professional for business and advanced users. Its colorful Luna interface and the Bliss desktop background became instantly recognizable.

The main mix-up is with Windows 2000 and Windows Me, both of which came earlier, in 1999 and 2000 respectively. Windows Vista came later, in 2007. Windows XP also had an unusually long life: mainstream support ended in 2009, while extended support for most editions ended on April 8, 2014.