Can't open Volume Controls

Ok, I'm using Windows XP with the service packs and everything. Ever since my brother put a load of Command & Conquer on and off the PC one day I haven't been able to open the (Advanced) Volume Controls. I can open the one-click miniture one from the tray, but not the one that controls inputs, etc. too.
Or rather, it opens but doesn't show anything other than that the taskbar thinks it's open.

I've tried replacing the file and opening it different ways, as well as Googling for answers and trying Windows Help Centre, but nothing works (especially Windows Help Centre).

Any idea what to do?