Why is my scroll button doing this?

'bout two months ago my “real” computer’s hard drive crashed, and I have yet to rectify the problem. In the meantime I’ve been using a three year old laptop. As the title cryptically suggests, it has a scroll button. Sometimes when I’m reading something like a long thread here, I’ll hit the scroll down button, and the scrollbar will do as I want it to. Then, for apparently no reason, clicking it again will cause the scroll bar to jump back to the top of the page. It’s really quite frustrating, since I press it by force of habit and thus often lose my place.

There are incidents, however, where I can understand why. Today I opened winamp, and once the program came up, the scroll button went from acting normally to being a hotkey to take me back to the top.

Why the hell is my computer doing this, and how can I get it to stop? Is it some kind of thing that the current settings think I WANT to do and there’s some way I’m triggering it? Or is my machine just messed up?