Just Follow the simple steps....
1) Go to the start menu and type 'taskschd.msc' in search box
3) Give the name for New task as 'Show Desktop'.
4) Select the Windows 8 option from 'Configure for'
5) Select the 'Triggers' tab and click the 'new button'.
6) Change the Begin the task Option to 'At log on' and click Ok
7) Select the 'Actions' tab and click the New button
8) Type C:\Windows\Explorer.exe on Program/Script field and click OK.
Then Restart your System and Enjoy the Changes.....................