From 
My digital Life:
When opening Windows Explorer on Windows 7 (Windows Explorer shortcut can be on the Start Menu or pin to Taskbar), the default folder opened by Windows 7 is “Libraries”
For user who doesn’t actually use Libraries, or prefer Windows  Explorer to open up My Documents, Documents, or even Computer (the  highest level view that shows all drives available on the computer),  this trick will set Windows Explorer to open and show the prefer folder  by default. 
- In Windows 7 Start Menu, click on the Windows Explorer shortcut (typically store inside All Programs -> Accessories) and then select Properties. On Windows 7 Taskbar, hold down Shift key, and then right click on the Windows Explorer icon, and then select Properties. If you already have one or more Windows Explorer windows open, right click on the Windows Explorer icon, and then right click on the Windows Explorer link again to click on Properties.
 
- In the Target text box under Shortcut tab, change the value to one of the following to open either Documents or Computer by default. 
My documents:  %windir%\explorer.exe Libraries\Documents
My Computer:   %windir%\explorer.exe /E,::{20D04FE0-3AEA-1069-A2D8-08002B30309D} 
Explorer.exe Command Line Syntax
%SystemRoot%\explorer.exe [/n][/
e][,/root],
X,[[/Select],
Y]
X specifies the object, and optionally with sub-object 
Y.
/
e switch  showes the left Windows Explorer tree view navigation pane together with  the right pane in list view, while /n hides the left navigation pane.
When the /root parameter is present, Explorer.exe will explore the root  object (X) and objects belonging to X. On the other hand, when the  /root switch is not present, Explorer.exe explores the object X, its  children, and other Explorer objects as well.
/Select switch puts the  focus on a file or folder.
For example:
%SystemRoot%\explorer.exe /N,%WinDir%\System32,/Select,%WinDir%\System32\Ping.exe
Command aboves will explore the \Windows\System32 folder and put the focus on the ping.exe program.
Tip: Normally, there is no need to specify full path to explorer.exe,  which is stored in the Windows folder, obtainable through the  environment variable WinDir, as the path already been defined in PATH  environment variable, and will be search through accordingly. As such,  Explorer will suffice to run the Windows Explorer shell.