var isIE5 = navigator.userAgent.toUpperCase().indexOf("MSIE 5") != -1;

var targetElementID = "contentDiv"; //, targetElementStyleOffset = 30 + 4 + 2;

function adjustHeight() {
    var fot = document.getElementById('footerDiv');
    var xx = fot.offsetHeight + 4;
    if (document.getElementById) {
        var targetElement = document.getElementById(targetElementID), documentHeight, totalOffset;

        if (targetElement) {
            documentHeight = document.documentElement.offsetHeight;
            if (targetElement.offsetHeight < (documentHeight - targetElement.offsetTop - 4)) {
                if (isIE5)
                    totalOffset = targetElement.offsetTop;
                else
                    totalOffset = targetElement.offsetTop + xx;

                targetElement.style.height = String(documentHeight - totalOffset) + 'px';
            }
        }
    }
}

window.onresize = adjustHeight;
window.onload = adjustHeight;