Windows Longhorn was designed to be the successor to Windows XP. It was scheduled for release in 2003, and the name "Longhorn" fits in well with it's purpose. Longhorn is the name of a bar that sits between "Whistler" (XP's codename), and "Blackcomb" (7's codename).
Due to the fact that Longhorn just kept implementing technologies that were supposed to be saved for Windows 7 (something known as "feature creep"), Microsoft basically reset development of Vista/Longhorn in August 2004.

These are screenshots of the earliest build that I was able to get my hands on and make work.

The splash screen is identical to the one from Windows XP Professional, except for a few slight differences (like the copyright date)

Apart from a new, shiny, color scheme and the fact that the date and time is now displayed, the login screen is identical to XP's

Most of the arrangement of things on the desktop has remained the same from XP. The only real difference is a slightly updated theme. No more Bozo the Clown as the default! That alone is a significant improvement.

The start menu is the same as the one from Windows XP, but with colors applied to it.