1) Right-click on the Windows Live Mail icon and then click Properties.
2) In the Windows Live Mail Properties window, click the Compatibility tab. Here, set the Compatibility mode to Windows Vista SP2. Click OK.
3) Run Windows Live Mail and you will see it's icon in the notification area (system tray). Right-click it's icon and choose Hide window when minimized.
Now whenever you minimize the Windows Live Mail window, it wont appear in the super bar and you can make it appear again by double-clicking its icon in the system tray.
For complete information with images, see here: Force Windows Live Mail To Always Minimize To The System Tray