I get a lot of questions about this. In Word 6 and 7, you can slow scrolling a bit by dragging the mouse just below the document window, rather than further down the screen. That trick doesn't help on fast Pentium machines, though; Word still scrolls too fast for comfort.
Fortunately, there's a much better way to handle selecting large blocks of text; the technique works in every word processor because it's a basic Windows function.

Click where you want the text selection to begin. Use the scroll bar instead of the cursor keys to move down in the document until you can see the end of the desired selection. Hold down the <Shift> key, then click at the end of the text you want to highlight. That's all you have to do: you won't be blinded by Word's scrolling speed again.