Whistler was the codename of this Microsoft Operating System.

Whistler was the codename for the Microsoft operating system that eventually became known as Windows XP. Windows XP was released in 2001 and was a significant upgrade from its predecessor, Windows 2000. It introduced a more user-friendly interface, improved stability, better performance, and enhanced multimedia capabilities. Windows XP became one of Microsoft's most popular and widely used operating systems, remaining in mainstream support until 2009 and extended support until 2014.