WinKey+M will minimize all open windows to the taskbar.
WinKey+Shift+M will restore all windows to their previous state.
WinKey+D can be used to toggle between the above conditions. This does the same
as "Show Desktop" icon.
Note that in all cases, the windows are not being maximized by these shortcuts,
they are only restored to their previous state.
Here's a complete list of keyboard shortcuts:
List of the keyboard shortcuts that are available in Windows XP
http://support.microsoft.com/kb/301583