if (document.all) {  
    window.attachEvent('onload', URL_To_Link);
}else {  
    window.addEventListener('load', URL_To_Link, false);
}

function URL_To_Link(){
    var a=document.getElementById("main").innerHTML;
    my=new RegExp("([^\"'])(http://[A-Za-z0-9\./=\\?%\\-&_+;]+)","gi");  
    str=a.replace(my,"$1<a target=_blank href=$2>$2</a>");
    document.getElementById("main").innerHTML=str;
}