Microsoft put far more work into the look and themes of Windows 7 than its previous operating systems. Pull down that design work into your non-7-system with Digital Inspiration's simple work-arounds.
You can't just grab the files from Windows 7's themes gallery and set them as your own. You can, however, use tools like 7-Zip and John's Background Switcher to pull out each theme pack's files, install them, set them to rotate (a la Windows 7), and even get the sounds, cursors, and other elements running.
It's a bit more work than "Click to install," but it's also a new source of themes, wallpapers, and other modifications that are more than compatible.
Download and Install Windows 7 Themes on your XP or Windows Vista Computer [Digital Inspiration]