var start = navigator.userAgent.indexOf("MSIE") + 5;
var end = start + 1;

Prototype.Browser.IE6 = Prototype.Browser.IE && parseInt(navigator.userAgent.substring(start, end)) == 6;
Prototype.Browser.IE7 = Prototype.Browser.IE && parseInt(navigator.userAgent.substring(start, end)) == 7;
Prototype.Browser.IE8 = Prototype.Browser.IE && parseInt(navigator.userAgent.substring(start, end)) == 8;