[jQuery] scrolling height of the window and placing popup windows

[snitch]

Hello,

for opening a PopUp Window I use the Popup.js from Adrian "yEnS" Mato
Gondelle.
Adrian has written a function to center the popup by calculating the
window height/with and popup width/height as well ans putting the
coord in the middle of these coordinates. Like this:

function centerPopup(){
        //request data for centering
        var windowWidth = document.documentElement.clientWidth;
        var windowHeight = document.documentElement.clientHeight;
        var popupHeight = $("#popupContact").height();
        var popupWidth = $("#popupContact").width();
        //centering
        $("#popupContact").css({
                "position": "absolute",
                "top": windowHeight/2-popupHeight/2,
                "left": windowWidth/2-popupWidth/2
        });
        //only need force for IE6

        $("#backgroundPopup").css({
                "height": windowHeight
        });

}
My Problem with this is, that it only centers the Popup correctly if
you haven't scrolled down the page. What I need is a jQuery way to get
the height of the actually scolled position to calculate the correct
value for the top css parameter as well, to get my PopUp every time in
the correct top/left position without having this scrolling bug.

Is there a way in jQuery how I can get the scrolled down coordinates?
I need the height of the scrolled position and by adding the half
height of the screen resolution I have the css top value which I need
here.

Best Regards, May someone can help me?

Marcel