function abrir_compra(){
	ancho=550;
	alto=610;
	pf=window.open ("compra.php","compra","width="+ancho+",height="+alto+",status=no,toolbar=no,menubar=no,location=no,resizable=yes");
	pf.moveTo(window.screen.availWidth/2-ancho/2, window.screen.availHeight/2-alto/2);	
}