if (/MSIE (\d+\.\d+);/.test(navigator.userAgent)) { //test for MSIE x.x;
    var ieversion = new Number(RegExp.$1) // capture x.x portion and store as a number
    if (ieversion <= 6)
      document.write("<div style='border: 1px solid #F7941D; background: #FEEFDA; text-align: center; clear: both; height: 75px; position: relative; z-index:5000'> <div style='position: absolute; right: 3px; top: 3px; font-family: courier new; font-weight: bold;'><a href='#' onclick='javascript:this.parentNode.parentNode.style.display=\"none\"; return false;'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-cornerx.jpg' style='border: none;' alt='Fechar esse alerta'/></a></div> <div style='width: 740px; margin: 0 auto; text-align: left; padding: 0; overflow: hidden; color: black;'> <div style='width: 75px; float: left;'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-warning.jpg' alt='Warning!'/></div> <div style='width: 275px; float: left; font-family: Arial, sans-serif; color:#000'> <div style='font-size: 14px; font-weight: bold; margin-top: 12px; color:#000'>Voc&ecirc; est&aacute; usando um navegador desatualizado</div> <div style='font-size: 12px; margin-top: 6px; line-height: 12px; color:#000'>Para que este site seja melhor visualizado, por favor, atualize seu navegador.</div> </div><div style='width: 75px; float: left;'><a href='http://www.firefox.com' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-firefox.jpg' style='border: none;' alt='Get Firefox 3.5'/></a></div><div style='width: 75px; float: left;'><a href='http://www.browserforthebetter.com/download.html' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-ie8.jpg' style='border: none;' alt='Get Internet Explorer 8'/></a></div> <div style='width: 73px; float: left;'><a href='http://www.apple.com/safari/download/' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-safari.jpg' style='border: none;' alt='Get Safari 4'/></a></div> <div style='float: left; width: 73px;'><a href='http://www.google.com/chrome' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-chrome.jpg' style='border: none;' alt='Get Google Chrome'/></a></div><div style='float: left;'><a href='http://www.opera.com/' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-opera.jpg' style='border: none;' alt='Opera'/></a></div> </div></div>");
}

//código que estava...
//var ie6_warning = navigator.userAgent.indexOf('MSIE 6')

//if (ie6_warning > 0)
//{
//    document.write("<div style='border: 1px solid #F7941D; background: #FEEFDA; text-align: center; clear: both; height: 75px; position: relative; z-index:5000'> <div style='position: absolute; right: 3px; top: 3px; font-family: courier new; font-weight: bold;'><a href='#' onclick='javascript:this.parentNode.parentNode.style.display=\"none\"; return false;'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-cornerx.jpg' style='border: none;' alt='Fechar esse alerta'/></a></div> <div style='width: 740px; margin: 0 auto; text-align: left; padding: 0; overflow: hidden; color: black;'> <div style='width: 75px; float: left;'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-warning.jpg' alt='Warning!'/></div> <div style='width: 275px; float: left; font-family: Arial, sans-serif; color:#000'> <div style='font-size: 14px; font-weight: bold; margin-top: 12px; color:#000'>Voc&ecirc; est&aacute; usando um navegador desatualizado</div> <div style='font-size: 12px; margin-top: 6px; line-height: 12px; color:#000'>Para que este site seja melhor visualizado, por favor, atualize seu navegador.</div> </div><div style='width: 75px; float: left;'><a href='http://www.firefox.com' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-firefox.jpg' style='border: none;' alt='Get Firefox 3.5'/></a></div><div style='width: 75px; float: left;'><a href='http://www.browserforthebetter.com/download.html' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-ie8.jpg' style='border: none;' alt='Get Internet Explorer 8'/></a></div> <div style='width: 73px; float: left;'><a href='http://www.apple.com/safari/download/' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-safari.jpg' style='border: none;' alt='Get Safari 4'/></a></div> <div style='float: left; width: 73px;'><a href='http://www.google.com/chrome' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-chrome.jpg' style='border: none;' alt='Get Google Chrome'/></a></div><div style='float: left;'><a href='http://www.opera.com/' target='_blank'><img src='http://info.template-help.com/files/ie6_warning/ie6nomore-opera.jpg' style='border: none;' alt='Opera'/></a></div> </div></div>");
//}

//código que encontrei...
//if (/MSIE (\d+\.\d+);/.test(navigator.userAgent)){ //test for MSIE x.x;
// var ieversion=new Number(RegExp.$1) // capture x.x portion and store as a number
// if (ieversion>=8)
//  document.write("You're using IE8 or above")
// else if (ieversion>=7)
//  document.write("You're using IE7.x")
// else if (ieversion>=6)
//  document.write("You're using IE6.x")
// else if (ieversion>=5)
//  document.write("You're using IE5.x")
//}
//else
// document.write("n/a")