Styler

This application allows you to do a lot of interesting customizations, though it is not easy to use at first.
It can add a drop shadow to your windows, change Clear Type contrast, change a visual style's color or a wallpaper's and much more ...
But its most useful function is located in the "Styler Toolbar" which can entirely change the look and feel of the Windows XP's toolbar. So, you can change your toolbar to make it look like Windows Vista's or Mace OSX's.
Styler doesn't use much memory (less than 1MB when it's used in the background), but to use all its function, especially for the drop shadow, you'll need a powerful computer.