I've been trying to find some resources to help you with that, and here's what I've come up with so far.
Create a banner with PowerPoint 2007 - this is a video of the steps you need to take to create a banner in PowerPoint.
http://cooltext.com/ - you choose one of the starting point designs seen below, then modify the settings to suit your needs. Creates text with background, no imported images allowed.
http://makefreebanners.com/ - this site gives you a choice of 150 background images to which you add your own text.