There are two ways to minimize all of your open windows at once.
Click the Show Desktop icon on the Quick Launch toolbar. You can click it again to reopen all the windows.
If you can't see the Show Desktop icon, try displaying more of the Quick Launch toolbar by clicking the double chevron icon . If that doesn't work, you might need to add the Quick Launch toolbar by right-clicking an empty area on the taskbar, pointing to Toolbars, and then clicking Quick Launch so that it has a check mark next to it.
Press the Windows logo key + M.