Does it need to be computer-drawn? The simplest solution might be to draw it on paper and then scan or photograph it into an image file. At least, that would be my first choice. Drafting images on the computer is a real pain in the butt if you don't have experience with it (at least, that's what I've found).
Software-wise, I've used both Photoshop and Illustrator to draft line drawings...Photoshop Elements might be able to do it and is a lot cheaper.
I hope that helps...if you get stuck, email me a photo of the drawing and I'll use Illustrator to turn it into a graphic for you!