function Is() {
  var agent  = navigator.userAgent.toLowerCase();
  this.major = parseInt( navigator.appVersion );
  this.ns    = !document.all;
  this.ns4   = ( this.ns && (this.major >= 4) );
  this.ie    = document.all;
  this.ie4   = ( this.ie && (this.major >= 4) );
  this.ie5   = ( this.ie && (this.major >= 5) );
  this.win   = ( agent.indexOf('win') != -1 );
  this.mac   = ( agent.indexOf('mac') != -1 );
  this.unix  = ( agent.indexOf('x11') != -1 );
  this.ie5   = ( agent.indexOf('msie 5') != -1);
  this.ieWin = (( this.win && this.ie ) || ( this.mac && this.ie5));
  this.nsWin = ( this.win && this.ns );
}