function checkOS() {
    var agt=navigator.userAgent.toLowerCase();

    // BROWSER
    var major = parseInt(navigator.appVersion);
    var nav  = ((agt.indexOf('mozilla')!=-1) && (agt.indexOf('spoofer')==-1)
                && (agt.indexOf('compatible') == -1) && (agt.indexOf('opera')==-1)
                && (agt.indexOf('webtv')==-1) && (agt.indexOf('hotjava')==-1));
    var nav4 = (nav && (major == 4));
    var nav4up = (nav && (major >= 4));
    var nav6 = (nav && (major == 5));
    var nav6up = (nav && (major >= 5));

    var ie     = ((agt.indexOf("msie") != -1) && (agt.indexOf("opera") == -1));
    var ie3    = (ie && (major < 4));
    var ie4    = (ie && (major == 4) && (agt.indexOf("msie 5")==-1) );
    var ie4up  = (ie  && (major >= 4));
    var ie5    = (ie && (major == 4) && (agt.indexOf("msie 5.0")!=-1) );
    var ie5_5  = (ie && (major == 4) && (agt.indexOf("msie 5.5") !=-1));
    var ie5up  = (ie  && !ie3 && !ie4);
    var ie5_5up =(ie && !ie3 && !ie4 && !ie5);

    // PLATFORM
    var win   = ( (agt.indexOf("win")!=-1) || (agt.indexOf("16bit")!=-1) );
    var mac    = (agt.indexOf("mac")!=-1);
    var linux = (agt.indexOf("inux")!=-1);

		if (ie && !ie3) cssSize="ie-win";
		else if (win && nav4up) cssSize="nn4-win";
		else if (mac && nav6up) cssSize="nn6-mac";
		else if (mac && nav4up) cssSize="nn4-mac";
		else if (win && ie3) cssSize="ie3-win";
		else if (linux && nav4up) cssSize="nn4-lnx";
		else cssSize="ie-win"; // default for everyone else
		
		return cssSize;
}