A Question

When I'm scrolling down a webpage, I often like to click in the middle of the scroll bar. Usually, that makes the webpage scroll down by exactly the length of the screen, so that the new material appears right at the top of the screen. But my Windows XP computer, for some unknown reason, actually scrolls down two screens' length with each click. This is very annoying, because then I have to carefully scroll back up and figure out where I left off reading. How can I make it stop?