I, too am a huge fan of that button.
Do you have the quicklaunch bar enabled? if not, then just right-click an
empty area of the taskbar, select toolbars>quicklaunch.
If you do have the quicklaunch bar enabled, but the 'show desktop' button is
gone, the problem takes a bit more effort, but is not hard to fix. Just go
to this page for instructions on re-creating the shortcut:
http://www.theeldergeek.com/restore_show_desktop_icon.htm.
FYI: In Windows XP, there are a host of new keyboard shortcuts that use the
Windows key on your keyboard (the one between ctrl and alt - it looks like a
Windows logo). One of these new shortcuts is "show desktop" - simply press
the Windows key and the letter "D" simultaneously. Two of my other favorites
are: Windows+E - launch explorer (not new to XP) and Windows+L - lock your
workstation.
Dan DeStefano