Edit the registry at:

HKEY_CURRENT_USER\Control Panel\Desktop\WheelScrollLines

This type REG_SZ value has a range is 0 - 99 and the default is 3. This entry determines the number of lines scrolled for each rotation of the mouse wheel on a Microsoft IntelliMouse™ when no modifier keys (such as CTRL or SHIFT) are pressed. If the value of this entry is 0, the screen will not scroll when the mouse wheel is turned. If the value of this entry is greater than the number of lines visible in the window, the screen will scroll up or down by one page. To direct Windows NT to interpret all wheel rotations as page-up or page-down commands, set the value of this entry to 99.