This tutorial uses PSP7, but could easily be done in PSP6. This is an idea tutorial, I supply the basic instructions and you take it from there, using your ideas and imagination. My images are copyrighted to me, but anything you create from this tutorial is yours alone.

Open a new image, 350x350, transparent background, 16 mil colors.
Double click the selection tool and use these setting.

Change your foreground to a gradient , click here to choose your gradient and choose a nice rainbow, linear gradient, 0 angle, 0 repeats, with the red at the top.

Flood fill the selection with your rainbow gradient.

Your image should now look like this.

Selections/select none to turn off your selection.

Effects/Geometric Effects/Circle

Effects/Blur/Gaussian Blue and use these settings

Here is your easy rainbow!
From here you can save it as a gif, tube it, deform it into any traditional rainbow shape and layer it onto another image, use it for buttons, banners, backgrounds, etc. The possibilities are endless, only limited by your imagination.