When you right-click on the Windows desktop, there’s an option to change the default size of desktop icons - you could either choose medium, large or the default “Classic” size.
Now if you are looking for something extra and don’t want to limit yourself to these three default sizes, try this trick:
Left-click anywhere on the desktop, hold the ‘Control’ key and move the scroll wheel on the mouse - the desktop icons will grow smaller or larger depending upon the direction of your scroll.
Uncheck the “Auto Arrange” option for some more interesting results. Now I use this on a Windows Vista desktop but the trick should probably work on XP systems as well (sorry, its a Vista specific feature).