<script>
function getInternetExplorerVersion()
// Returns the version of Internet Explorer or a -1
// (indicating the use of another browser).
{
  var rv = -1; // Return value assumes failure.
  if (navigator.appName == 'Microsoft Internet Explorer')
  {
    var ua = navigator.userAgent;
    var re  = new RegExp("MSIE ([0-9]{1,}[\.0-9]{0,})");
    if (re.exec(ua) != null)
      rv = parseFloat( RegExp.$1 );
  }
  return rv;
}
function checkVersion()
{
  var msg = "You're not using Internet Explorer.";
  var ver = getInternetExplorerVersion();
 
  if ( ver > -1 )
  {
    if ( ver >= 6.1 ) 
      msg = "You're using a recent copy of Internet Explorer."
    else
      msg = "You should upgrade your copy of Internet Explorer.";
  }
  alert( msg );
}
</script>