Windows 7 (if you use Windows 7, this is what you can do)
Here are official instructions from Microsoft
http://windows.microsoft.com/is-IS/wind ... ing-themes
Here is a quick and dirty verstion
Right click on your desktop
Choose Personlize
Scroll down until you see "Windows 7 Basic" or you can try other themes in there and see if they are good.
After clicking the theme..your screen will flicker and it will change to the one you clicked.