On my ThinkPad R500 running Windows Vista 64-bit, in the Mouse control panel, I have Vertical Scrolling set to scroll 3 lines per notch, which is the default. At random times, it gets changed to 1 line per notch, and I have to reset it. It usually seems to happen when I'm doing something (i.e., I don't typically come back to the computer after being away and find that it changed while I was gone) but to my knowledge I'm not doing any single thing consistently to make it happen. The problem does not happen on my desktop rig, which is also running Vista 64.
What causes this, and how can I prevent it from happening?