Do you mean your wallpaper/background? That can be changed by right clicking on your desktop and going to display properties. Tell it under wallpaper that you want to stretch the image. That will make it fill the screen.
If you've got vista, looks like you choose "personalise" when you right click on the desktop, then it brings you to choices. Click on whatever it is you want to change!
Does that help?? Disregard if I've totally misunderstood!