I recently switched to Vista and at the same time registered witnniTunes. My USB earphones worked fine for a while till I registered with iTunes, and now they all have this traffic cone logo and nothing will play on my earphones.
The first thing that comes to my mind is that iTunes may have hijacked one or more of your 'speakers/headphones' settings. Why don't you take a careful look through the iTunes Preferences and Options sections, (and see if there's one that configures which app controls the speakers/headphones for your system).
Then, go into your system's settings for your headphones, and if they were unselected, reselect them.