You could use just about any kind of paint program to create backgrounds. If you're looking for a repeating texture tile to use, Texture Maker is a great prog (albeit a little overwhelming to use at first). For a full blown graphical editor like Adobe Photoshop, you could use the Gimp for Windows. It's extremely powerful, and totally free!
http://www2.arnes.si/~sopjsimo/gimp/
You could also simply draw the background you want on paper and then scan it into a paint program for color and touch-up.
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