Hm, only 2 ways I can think of doing it.
Either use a java applet which does the banner switching on it's own, or the prefered method, which is create a frame with the banner in it and use a dhtml timed refresh inside that frame.
I believe it would refresh the frame page, without refreshing the main content page itself.
An excellant example of this is http://www.station.sony.com/chronx/
The entire Sony Station site handles their adds this way.
Occasionally they add the annoyance of a pop-up window, however.
Might study their code to get an idea of that way of handling it.
