function navigate_cat(drop) {

    var query = window.location.search.substring(1);
    if (query.length > 0)
    {
        var paramFound = false;
        var newSearch = "";
        var parms = query.split('&');
        for (var i=0; i<parms.length; i++) {
            var pos = parms[i].indexOf('=');
            if (pos > 0) {
                var key = parms[i].substring(0,pos);
                var val = parms[i].substring(pos+1);
                
                if (key == "cid")
                {
                    paramFound = true;
                    val = drop.options[drop.selectedIndex].value;
                }  
                
                newSearch += ((newSearch.length == 0) ? "" : "&") + key + "=" + val;               
             }
        }
        
        if (!paramFound)
            newSearch += "&cid=" + drop.options[drop.selectedIndex].value;
        
        window.location.href = window.location.href.replace(window.location.search.substring(1),"") + newSearch;
    }
    else
        window.location.href = window.location.href.replace("cid=","") + "?cid=" + drop.options[drop.selectedIndex].value;
        
        
}