Unfortunately, there is no way to fix this. Microsoft decided to strip this functionality from Windows Vista and above. However, there is a trick that still works:
Hold the "Windows Key" and press M to minimize all windows. It will minimize everything except Outlook on the Desktop, essentially achieving the same effect as Show Desktop. "Windows Key" + Shift + M will restore your minimized windows.
Hold the "Windows Key" and press M to minimize all windows. It will minimize everything except Outlook on the Desktop, essentially achieving the same effect as Show Desktop. "Windows Key" + Shift + M will restore your minimized windows.