http://support.microsoft.com/kb/314853/en

The options that you can use with Explorer.exe are /n, /e, /root (plus an object), and /select (plus an object).
   Option            Function
   ----------------------------------------------------------------------
   /n                Opens a new single-pane window for the default
                     selection. This is usually the root of the drive that
                     Windows is installed on. If the window is already
                     open, a duplicate opens.

   /e                Opens Windows Explorer in its default view.

   /root,<object>    Opens a window view of the specified object.

   /select,<object>  Opens a window view with the specified folder, file,
                     or program selected.

   Examples
   -----------------------------------------------------------------------
   Example 1: Explorer /select,C:\TestDir\TestProg.exe
              Opens a window view with TestProg selected.

   Example 2: Explorer /e,/root,C:\TestDir\TestProg.exe
              Opens Explorer with drive C expanded and TestProg selected.

   Example 3: Explorer /root,\\TestSvr\TestShare
              Opens a window view of the specified share.

   Example 4: Explorer /root,\\TestSvr\TestShare,select,TestProg.exe
              Opens a window view of the specified share with TestProg selected.
