Edit in JSFiddle

window.onload = function() {
  var oDiv = document.getElementById('div1');

  oDiv.onmousedown = function(ev) {
    var ev = ev || event;
    var disX = ev.clientX - this.offsetLeft;
    var disY = ev.clientY - this.offsetTop;

    document.onmousemove = function(ev) {
      var ev = ev || event;
      oDiv.style.left = ev.clientX - disX + 'px';
      oDiv.style.top = ev.clientY - disY + 'px';
    }

    document.onmouseup = function() {
      document.onmousemove = document.onmouseup = null;
    }
    return false;
  }
}

<div id="div1"></div>

#div1 {
  width: 100px;
  height: 100px;
  background: red;
  position: absolute;
}