Which Windows version was code-named 'Whistler'?

Windows XP was code-named “Whistler” during development. Microsoft selected the name after Whistler, British Columbia, where many employees visited the Whistler-Blackcomb ski resort for company retreats and design meetings.

Whistler emerged when Microsoft combined two earlier projects: Neptune, intended as a consumer-oriented successor to Windows Me, and Odyssey, intended to succeed Windows 2000. Bringing them together allowed Microsoft to create one NT-based operating system for both home and business users instead of maintaining separate consumer and professional development paths.

The Whistler project was publicly shown through preview builds during 2000 and early 2001. Microsoft officially unveiled the final name, Windows XP, on February 5, 2001; “XP” referred to “experience.” Windows XP reached manufacturing on August 24, 2001, and general retail availability on October 25. Longhorn, by contrast, was the codename associated with Windows Vista.