navig = 
  (((navigator.appName == "Netscape") && 
  (parseInt(navigator.appVersion) >= 3 )) || 
  ((navigator.appName == "Microsoft Internet Explorer") && 
  (parseInt(navigator.appVersion) >= 4 ))); 

function precharge(img){
    var a=new Image(); a.src="images/navig/"+img; return a; 
}

if(navig) {
    home = precharge('home.gif');
    home_on = precharge('home_a.gif');
    valuep = precharge('valuep.gif');
    valuep_on = precharge('value_a.gif');
    search = precharge('search.gif');
    search_on = precharge('search_a.gif');
    parts = precharge('parts.gif');
    parts_on = precharge('parts_a.gif');
    source = precharge('source.gif');
    source_on = precharge('source_a.gif');
    where = precharge('where.gif');
    where_on = precharge('where_a.gif');
    links = precharge('links.gif');
    links_on = precharge('links_a.gif');
    iso = precharge('iso.gif');
    iso_on = precharge('iso_a.gif');
    about = precharge('about.gif');
    about_on = precharge('about_a.gif');
}

function imgOn(imgname){
    if(navig) document.images[imgname].src = eval(imgname+"_on").src;
}

function imgOff(imgname){
    if(navig) document.images[imgname].src = eval(imgname).src;
}