scrolling in the wrong direction

If I have two word docs open at once (side-by-side, for comparison
purposes, in Normal view) when I scroll with my mouse, often it scrolls
vertically on one document but horizontally on the other. I want to be
able to scroll veritcally on both documents.

I tried going into tools -- options and disabling the horizontal
scroll bar, but this disabled scrolling altogether on both documents.
Vertical scrolling works fine in other apps, such as internet explorer.
Has anyone experienced this and/or found a fix for it? Thanks.