function openPopup(url, width, height, params) {
	
	if (screen) {
		var xMax = screen.width, yMax = screen.height;
	}
	else {
		if (document.layers) {
			var xMax = window.outerWidth, yMax = window.outerHeight;
		} else { 
			var xMax = 640, yMax=480;
		}
	}
	
	var xOffset = (xMax - width)/2, yOffset = (yMax - height)/2;
	var str = 'width='+width+',height='+height+',screenX='+xOffset+',screenY='+yOffset+',top='+(yOffset-30)+',left='+xOffset+''
	
	if (params) {
		str += ','+params;  
	}
	
	if (window._popupWindow) {
		window._popupWindow.close();
	}
	
	window._popupWindow = window.open(url, '_popupWindow', str); 
	
	if (window._popupWindow) {
		window._popupWindow.focus();
		return false;
	}
	
	return true;

}