below are step by step instructions to help you out.
1. go onto the desktop and right-click
2. scroll down to properties
3. click on the desktop tab.
4. assuming that you havent changed the color in the color box (on the right, below the "position" tab) scroll up to the top of the background options and you should see one that says "none"
5. click none (at the top of the background options)
6. voila! your wallpaper is now the original windows background!