showModalDialog

<script>
function fnRandom(iModifier){
   return parseInt(Math.random()*iModifier);
}
function fnSetValues(){
   var iHeight=oForm.oHeight.options[
      oForm.oHeight.selectedIndex].text;
   if(iHeight.indexOf("Random")>-1){
      iHeight=fnRandom(document.body.clientHeight);
   }
   var sFeatures="dialogHeight: " + iHeight + "px;";
   return sFeatures;
}
function fnOpen(){
   var sFeatures=fnSetValues();
   window.showModalDialog("showModalDialog_target.htm", "", 
      sFeatures)
}
</script>

<form name=oForm>
Dialog Height <select name="oHeight">
   <option>-- Random --
   <option>150
   <option>200
   <option>250
   <option>300
</select>

Create Modal Dialog Box
<input type="button" value="Push To Create" 
   onclick="fnOpen()">
</form>

http://msdn.microsoft.com/library/default.asp?url=/workshop/author/dhtml/reference/methods/showmodaldialog.asp