In Windows XP, you get a 'Show Desktop' shortcut button in your task bar by default.
This does not appear in Windows 7. How do I add a shortcut button in Windows 7 to show the desktop and hide the windows of all running applications?
Normally you should hover your mouse on the small rectangle on the right of the clock. It will trigger AeroPeek, which makes the windows transparent and let you see the desktop.
If you really want a "Show desktop" button on the taskbar, follow this tutorial.
You can also use Win + D.
can also use Win + M (minimizes all), and then Win + Shift + M (toggles them back)