Peace on Earth
I don't normally think much of one-wallpaper Windows themes, but Peace on Earth is particularly nice. The single image is a beautifully rendered photograph of Earth from outer space, with North America smack in the middle. The only other change this theme makes to your user interface is to use a cool blue for the windows colors.
To install the theme, just download and unzip the archive, then double-click the file; Windows takes care of the rest.
Peace on Earth is a simple little Windows 7 theme, and well worth the download.