var win = null; 
function popup(pagina,nome,w,h,sb,sz){ 
LeftPosition = (screen.width) ? (screen.width-w)/2 : 0; 
TopPosition = (screen.height) ? (screen.height-h)/2 : 0; 
settings ='height='+h+',width='+w+',top='+TopPosition+',left='+LeftPosition+',scrollbars='+sb+',resizable='+sz+',toolbar=no,location=no' 
win = window.open(pagina,nome,settings) 
}