function checkOS() {
  if(navigator.userAgent.indexOf('Linux') != -1)
    { var OpSys = "Linux"; }
  else if((navigator.userAgent.indexOf('Win') != -1) &&
  (navigator.userAgent.indexOf('95') != -1))
    { var OpSys = "Windows95"; }
  else if(navigator.userAgent.indexOf('Win') != -1)
    { var OpSys = "Windows3.1 or NT"; }
  else if(navigator.userAgent.indexOf('Mac') != -1)
    { var OpSys = "Macintosh"; }
  else { var OpSys = "other"; }
  return OpSys;
}

var OpSys = checkOS();

// convert all characters to lowercase to simplify testing 
var agt=navigator.userAgent.toLowerCase(); 
// *** BROWSER VERSION *** 
// note: on IE5, these return 4, so use is_ie5up to detect IE5. 
var is_major = parseInt(navigator.appVersion); 
var is_minor = parseFloat(navigator.appVersion); 

var is_fire = (agt.indexOf("firefox") != -1);
var versionindex = (navigator.userAgent.indexOf("Firefox")+8);
var is_fire1up = (is_fire && (parseInt(navigator.userAgent.charAt(versionindex))>=1));

var is_nav  = ((agt.indexOf('mozilla')!=-1) && (agt.indexOf('spoofer')==-1) 
            && (agt.indexOf('compatible') == -1) && (agt.indexOf('opera')==-1) 
            && (agt.indexOf('webtv')==-1));
var is_nav4 = (is_nav && (is_major == 4));
var is_nav4up = (is_nav && (is_major >= 4));
var is_navonly = (is_nav && ((agt.indexOf(";nav") != -1) || (agt.indexOf("; nav") != -1)) );
var is_nav5 = (is_nav && (is_major == 5));
var is_nav5up = (is_nav && (is_major >= 5));

var is_ie   = (agt.indexOf("msie") != -1);
var is_ie3  = (is_ie && (is_major < 4));
var is_ie4  = (is_ie && (is_major == 4) && (agt.indexOf("msie 5.0")==-1) );
var is_ie4up  = (is_ie  && (is_major >= 4));
var is_ie5 = (is_ie && (is_major == 4) && (agt.indexOf("msie 5.0")!=-1) );
var is_ie5up  = ((agt.indexOf("msie 5")>-1) || (agt.indexOf("msie 5.0")>-1) || (agt.indexOf("msie 5.5")>-1) || (agt.indexOf("msie 6")>-1) || (agt.indexOf("msie 7.0")>-1));
var is_ie7 = (is_ie && (is_major == 4) && (agt.indexOf("msie 7.0") != -1));

var isIE4, isIE4PC, isNav4PC, isNav4, isNav3;

if (is_ie4 || is_ie4up || is_nav5 || is_nav5up) {
        isIE4 = true;
        if (OpSys != "Macintosh") {
                isIE4PC = true;
        } 
} else if (is_nav4) {
        isNav4 = true;
        if (OpSys != "Macintosh") {
                isNav4PC = true;
        } 
} else if  (is_nav) {
        isNav3 = true;  
}