You can get a 30 day free trial of Adobe Photoshop CS2 or Adobe Photoshop Elements 4 on their website.
Adobe's website
Also, you could use Jasc Paint Shop Pro, Microsoft Digital Image Pro (DIP), Ulead Photo Impact (PI), Corel Draw or any kind of program that supports .png files & layers.
Just poke around on your computer and see what kind of photo editing software you might have.
But if you decide to buy- Photoshop Elements is awesome, has the capabilities of most things you would need to create banners, and is much less expensive than Photoshop CS2.