Go to Start > Shutdown. When the dialog appears, hold CTRL+ALT+SHIFT and press Cancel. Explorer will cleanly unload all of it’s resources and shutdown. To start it back up, open Task Manager (CTRL+SHIFT+ESC is one way) and go to File > New Task and run ‘explorer’.

This method was designed for people writing plugins and handlers for Explorer who needed to be able to unload it all and start fresh without rebooting or uncleanly killing Explorer’s process.