ok Steve, I fixed it. With your reply, I got the chance to install
IE Developer Toolbar, I didn't know that there was such a tool for IE (I do most of the work in Firefox and I just check the sites in IE when I finish each stage), and I traced the 40 px top-margin in the
main_vas.IE7.css style sheet. I couldn't trace that from Firebug, since this top-margin is located in another style sheet which is rendered only when IE is the browser. (a IE7 specific style sheet)
thanks for the help
