var shi = window.screen.availHeight;
var swi = window.screen.availWidth;

function show_snav(wd,hg,lnk){
if(!wd)wd=800
if(!hg)hg=600
var lf=(swi-wd)/2;
var tp=(shi-hg)/2;
var adp="";
win_snav=window.open(lnk,"win", "left=" + lf + ",top=" + tp+",width=" + wd + ",height=" + hg + adp);
win_snav.focus();
 
return false; 
}