Thank you, Preacher, you helped me a lot. Now I understand what happened (presuming that all the others who saw that bloody scroll did the same thing as you).
As already said, I've tested myself how it looks on 800x600, with two browsers and applying several XP themes. From your screenshot, I've realized one thing, though: when IE is not maximized, it shows the window border (6 pixels wide in XP default theme). So, if you resize its window to 800x600, the visible area of the page you're looking at it's smaller with about 12 pixels. When you are in 800x600 and have the window maximized, the window's border is hidden.
When I did my tests, I was at 800x600 screen resolution, and had the browsers maximized. When you did yours, you where at a higher resolution, and the window was not maximized, so you "lost" 12 pixels of the viewing area.
From my mainstream sites experience, I always consider 20 pixels of vertical scrollbar and add another 5 pixels just to be sure. Now I saw that I have to add more just to be sure
( boring long post, isn't it? )
Thanks again, Preacher.