The xpclient release branch, dedicated to XP, spun off from the main branch in 2001. This was likely spurred by Paul Thurrott's statement on his review of build 5048: 'The problem, I was told recently, was that the underpinnings of Longhorn-then based on the Windows XP code base-were struggling under the weight of all of the technologies that Microsoft planed to implement in this release.' However, pre-reset 'Longhorn' builds are based, not on XP, but on Windows Server 2003 code. The popular belief is that pre-reset 'Longhorn' builds are based on Windows XP.

Bill Gates would later state that this choice for a codename of the operating system was 'a bit random'. Whistler and Blackcomb are names of mountains in British Columbia, Canada, and Longhorn is the name of a saloon located in between the two mountains, representing the operating system's initial status as an internim release between the two products. 'Longhorn' was chosen as the codename for the operating system to represent its initial status as an internim release between Windows XP (codenamed 'Whistler') and 'Blackcomb'. 3 Concepts, presentations, demos and prototypes.