Why does my computer randomly change volume?

Factual Questions
1

I consider myself a pretty computer savvy guy, but this is some of the strangest behavior I’ve seen. I have winamp on pretty much 24/7, and so I often notice that the volume randomly changes. It’s quick, shifting from loud to soft, or vice versa, in less than a second. But it’s not just winamp–if another application is playing audio, its volume goes shifts at the same time; leading me to think that either windows system volume is changing, or something to do with my speakers, or something to do with my sound card settings is.

I have WinXP, a Soundblaster Audigy, and Creative Soundworks FPS2000 speakers attached to it via S/PDIF.

2

Winamp directly modifies the system volume. So an alteration to its volume affects everything.

Chief suspect is the mouse wheel. If it is moved when focus is on winamp, or if the system thinks it’s moved, this changes the volume. I do this accidentally quite often and it’s really annoying. Wish there was a way to switch it off.