You could make the scrollbar visible on all pages.
When the page is short enough, the scrollbar will have no effect:

html { 
    overflow: -moz-scrollbars-vertical; 
    overflow-y: scroll;
}

Source of that code: Making the main scrollbar always visible

I have a website consisting of a few pages. Some of them have vertical scroll bars and some of them not. The body on the pages without scrollbar is a bit wider than the on the pages with scrollbar (by the scrollbar’s width). Is there any way to set exactly the same width on both type of pages? That small difference in width causes some problems such as the some links in the top manue are in a bit different places. The same is with other elements. I would like to avoid it.