Adjusting volume on Studio 1537 causes full screened applications to minimize

This is only the case for applications which have an actual full screen feature, rather than just a maximized window. Technically, I guess, it's the blue volume gauge which causes them to minimize, rather than the change in volume itself. At first I thought it was an issue with Firefox and viewing YouTube videos (for example) in full screen, but I've noticed now that I have the same problem whilst watching DVDs using Windows Media Center. Even if I use the program's on-screen volume control as opposed to the laptop's touch sensitive buttons, the window is minimized, sometimes causing the program to crash. Whilst I have experienced the problem with Firefox for a while, it only affected Windows Media Center for the first time today, and now does so 100% of the time. (That said, I only began using my system to watch DVDs recently, but during the time that I'd been watching DVDs and adjusting the volume wasn't a problem, it seemed not to affect Firefox either.) I can't think of any system changes I've made which could have caused the issue. I am using Vista.