Open Desktop Background by clicking the Start button
, clicking Control Panel, clicking Appearance and Personalization, clicking Personalization, and then clicking Desktop Background.
Clickthe picture or color you want for your desktop background.
If the picture you want to use is not inthe list of desktop background pictures, click the Picture location down arrowto view other categories, or click Browse to search forthe picture on your computer. When you find the picture you want, double-clickit. It will become your desktop background and appear in the list of desktopbackgrounds.
Look for pictures in other locations on your computer
UnderHow should the picture be positioned, choose to have thepicture fit the screen, tile, or be centered on the screen, and then click OK.
Follow the bellow link and with the help of video can change easlly.http://www.bing.com/videos/watch/video/how-to-change-desktop-background-in-windows-vista/2e513abb49d87442bf852e513abb49d87442bf85-527728969217?q=CHANGE+BACK+GROUND+&FROM=LKVR5>1=LKVR5&FORM=LKVR1
Hope this helps ,