netscape problem mit abfragen und zuweisungen

[peri]

ich habs:
bitte schön:::::

var system= (navigator.appVersion.indexOf("Mac")==-1)?"PC":"MAC";
var browser=(navigator.appName.indexOf("Microsoft")==-1)?"NS":"IE";
var systembrowser=system+ browser;

var skinpath='skin/{skinname}/';
switch (systembrowser)
 {
  case "PCIE":
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ie_pc.css'>");
  break;
  case "PCNS":
  if(navigator.appVersion.substring(0,1) <="4")
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ns_pc.css'>");
  else
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ns6_pc.css'>");
  break;
  case "MACIE":
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ie_mac.css'>");
  break;
  case "MACNS":
  if(navigator.appVersion.substring(0,1) <="4")
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ns_mac.css'>");
  else
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ns6_mac.css'>");
  break;
  default:
  document.write("<link rel=stylesheet type='text/css' href='"+skinpath+"ie_pc.css'>");
  break;
}