function popup_window(tURL,nWidth,nHeight,tOptions) {
	var nTop, nLeft;
	nTop=parseInt(screen.availHeight/2-nHeight/2);
	nLeft=parseInt(screen.availWidth/2-nWidth/2);
	if ( typeof tOptions == 'undefined' )
		var tOptions='directories=no,location=no,menubar=no,scrollbars=no,resizable=no,status=no,toolbar=no';
	window.open(tURL,'_blank','top='+nTop+',left='+nLeft+',height='+nHeight+',width='+nWidth+','+tOptions);
}