get window to pop up in centre??

i an using a popup window script, but it always pops up on the top left! how would i get it to pop up in centre?
Here is the code in the head section of page:

Code:

---

<SCRIPT language="JavaScript">
<!--hide

function newwindow()
{
window.open('sendmessage.php','jav','width=550,height=400,resizable=no');
}
//-->
</SCRIPT>

---

And in the body section:

Code:

---

<br>"[<a href=\"javascript:newwindow()\" >Send Message</a>]&nbsp";

---

try this:

http://www.dynamicdrive.com/dynamicindex8/popwin.htm

good luck:)