You could go to a Kinko's and rent a Mac workstation. They have Illustrator installed.
Using a vector-based program like Illustrator will give you much better results than a raster-based program like Photoshop. Vector-based programs produce files that are constructed with vector points - they're mathematical formulas that allow your image (bumper sticker, in this instance) to scale infinitely. If you wanted to make a banner out of the same file, it would produce lines/edges that are just as crisp as those on the bumper sticker.
In a raster-based file, the image is made up of non-scalable pixels. If you scale an image, you'll just get bigger squares and jagged, fuzzy edges.
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