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 >= 7) {
        //ok
    } else if (ieversion >= 5) {
        alert("Your version of Internet Explorer is obsolete. This website will not operate in Internet Explorer 6 (or earlier versions).")
        window.location.href("/browsernotsupported.htm");
    }
}