function checkBrowser(altURL){

  var Netscape, MSIE, Opera, Unknown;
  var Win, Mac, Other;
  var NetscapeVer, MSIEVer, OperaVer;

  var 
  // detect browser
  
  Netscape = navigator.appName == "Netscape";
  MSIE = navigator.appName == "Microsoft Internet Explorer";
  Opera = navigator.userAgent.indexOf("Opera") > -1;
  Unknown = !(Netscape || MSIE || Opera);
  
  // detect platform
  
  Win = navigator.userAgent.indexOf("Win") > -1;
  Mac = navigator.userAgent.indexOf("Mac") > -1;
  Other = !(Win || Mac);
  
  // now extract version numbers
  
  if(Netscape) {
    NetscapeVer = parseFloat(navigator.appVersion);
  
  }else if(MSIE) {
    n = navigator.userAgent;
    MSIEVer = n.substr(n.indexOf("MSIE ")+("MSIE ").length, 4);
    MSIEVer = parseFloat(MSIEVer); 
    // converts it into a floatint point number

  }else if(Opera) {
      OperaVer = n.substr(n.indexOf("Opera ")+("Opera ").length, 4);
      OperaVer = parseFloat(OperaVer);    
  }

  if(Netscape) {
    
	if(NetscapeVer < 4)
      window.location = altURL; 
  
  }else if(MSIE) {
    
	if(Win && MSIEVer < 5.5)
	  window.location = altURL; 
	
	else if(Mac && MSIEVer < 5.1)
	  window.location = altURL; 
    
  }else if(Opera){
	if(OperaVer < 7)
	    window.location = altURL; 
  }

}