Minimize all open windows in Windows 7
If you have several windows open at one time, you can minimize them by using the Show Desktop icon on the Systray on your Taskbar. The Show Desktop minimizes all open windows so you can access your Desktop.
To re-open all previously minimized windows, click the 'Show Desktop' icon again and they will all reappear.
Also, to view the Desktop without actually minimizing all screens, you can use Peek. Hover your mouse pointer over the 'Show Desktop' icon without clicking. This way, you can view your desktop momentarily without minimizing anything.
Bonus Tip: You can also press the keyboard shortcut Windows Key + D to get to the Windows Desktop.