why is Windows Explorer opening in My Documents from Accessories and then opens in My Computer using the Windows icon key + e on my keyboard?
To explain this a bit more clearly...
The shortcut in Windows Start Menu (Accessories) for Windows Explorer is set so that it points to the user's “My Documents” (in Windows XP or earlier) or “Documents” (in Windows Vista) in the logged on user home directory. This is because Microsoft envisaged that most users will make use of the folder to manage their documents and files, thus having Windows Explorer opening in My Documents by default is convenient for most users. You can alter the default folder & view by editing the shortcut target.
However, because not everybody will want to go to My Documents, an alternative destination, My Computer, is provided and is available by pressing the Windows key then the E key. This one is not so easily editable - it seems to be hard coded.
To open Explorer in other folders or locations and to influence behaviour, e.g. what sidebar if any is shown, see the article I linked to, and also this one
http://www.mydigitallife.info/how-to-change-windows-explorer-default-folder/... and literally hundreds more which are only a Google search away.