Windows Vista displays sidebar with different gadgets such as contacts, notes, RSS reader etc. But if you are using the sidebar feature and you want to disable it starting after every windows starts, then you can do this through sidebar properties.
Right click the Windows Sidebar icon displayed in the system tray icons.
select properties from the menu list. This would display the following Windows Sidebar Properties window. Un mark the checkbox with label Start Sidebar when Windows starts. This would now prevent the sidebar starting when windows starts.