Windows Longhorn

The codename for Windows Vista. It featured a sidebar built into explorer.exe and was based on the Windows XP codebase up until Windows Longhorn Build 4093. From Windows Longhorn build 4008, explorer.exe had a severe memory leak and wasn't fully fixed until the development reset. The development was resetted in September 2004 and based on Windows Server 2003 SP1 codebase. The first Windows Longhorn build to feature the Windows Server 2003 SP1 codebase was Windows Longhorn Build 5048. It was slow and laggy. Windows Longhorn was a fucked up development project from Microsoft.

A lost Windows OS that is actually pretty good. It eventually got replaced by Vista.