Así de forma rápida, he hecho ésto. Funciona correctamente
<script language="javascript"> etc
HOLA!
<div id="ventana" onMouseDown="clicked=true" onMouseUp="clicked=false" style="position: absolute; left: 50px; top: 150px; background-color: orange; width: 150px; height:
var IE = document.all?true:false;
if (!IE) document.captureEvents(Event.MOUSEMOVE);
document.onmousemove = getMouseXY;
var tempX = 0;
var tempY = 0;
var clicked = false;
function getMouseXY(e) {
if(clicked){
if (IE) {
tempX = event.clientX + document.body.scrollLeft;
tempY = event.clientY + document.body.scrollTop;
} else {
tempX = e.pageX;
tempY = e.pageY;
}
if (tempX < 0){tempX = 0;}
if (tempY < 0){tempY = 0;}
document.getElementById("ventana").style["left"] = tempX - document.getElementById("ventana").offsetWidth/2;
document.getElementById("ventana").style["top"] = tempY - document.getElementById("ventana").offsetHeight/2;
}
}