Granny, you ask a question that I find interesting in the following sense: I don't know the answer. You certainly could watch a DVD and mute the sound while listening to some other music play. The problem is that the DVD will play the exact same patterns over and over. What I think you want are music visualizations -- programs that will analyze the music and create a unique visual display that is never the same. Wikipedia has a good article on them:
However, after reading that I get the impression that all the best visualizations are built into other programs such as WMP or WinAmp. What would be ideal is a visualizer that monitored your audio playback, no matter what the source. Since I know of no such thing, we are back to trying to get whatever player you are using to display a visualization. When you tune into your local radio stations streaming music, what is used to play the music? If it uses a plug-in that plays it from the web browser, there's still a chance that the playback could be routed to another stand-alone player that can handle that streaming format and also do visualizations.