The Windows Sidebar originated in Windows Longhorn build 3683. It was kept until development reset. However, the sidebar was made available for build 5219 as a separate download, it was also included in build 5212, but not enabled by default, and was not included in 5231. With build 5308 it was included again. In Windows 7, the Sidebar was replaced with Desktop Gadgets starting with build 6519. Desktop Gadgets were removed in Windows 8 and replaced with Live Metro tiles, since it became a security risk.
In pre-reset Longhorn builds, the Sidebar ran in explorer.exe, which caused massive memory leaks (notably with builds after build 4015), thus in post reset it ran in its own process, sidebar.exe.