function openwindow(url, windowname, width, height) 
{

	var options = "location=no, menubar=no, status=yes, scrollbars=yes, resizable=yes, toolbar=no";
	
	if (width) 
	{
		if (width < window.screen.width)
		{
			options += ", left=" + (window.screen.width-width)/2;
		}
  		else 
		{
  			width = window.screen.width;
		}
  		options += ", width=" + width;
 	}
 	
 	if (height) 
 	{
		if (height < window.screen.height)
		{
			options += ", top=" + (window.screen.height-height)/2;
		}
		else 
		{
			height=window.screen.height;
		}
		options += ", height=" + height;
 	}

	window.open(url, windowname, options);
}