strange mouse bug

Just in the last few days, my mouse has begun to act up strangely.
Whenever I highlight text, or go over it, a small black line appears below the mouse cursor. It's driving me crazy.

I suppose I ought to provide some more detail.
I'm on Vista x86, graphics card is a Radeon x800 (drivers up to date). The mouse is a basic optical mouse - Microsoft made, I'll add (or, rather, marketed).
"furquhart" wrote:

Just in the last few days, my mouse has begun to act up strangely.
Whenever I highlight text, or go over it, a small black line appears below the mouse cursor. It's driving me crazy.