<input type="text"     id="id" onchange='javascript:te("id");' size="50">

<script>

function te(obj){

    var value=document.getElementById(obj).value;

    var re=/(d+)(d{3})/;

    if(value.indexOf(",")==-1){

    var s=value.toString();

    if(s.indexOf(".")!=-1){

        var st = s.split(".");

        while(re.test(st[0]))st[0]=st[0].replace(re,"$1,$2");

        document.getElementById(obj).innerText=st[0]+"."+st[1];

    }else{

        while(re.test(s))s=s.replace(re,"$1,$2");

        document.getElementById(obj).innerText=s;

    }    

    

    }else{

    var s=value.toString();

    while(s.indexOf(",")!=-1){

        s=s.replace(",","");

    }

    if(s.indexOf(".")!=-1){

        var st = s.split(".");

        while(re.test(st[0]))st[0]=st[0].replace(re,"$1,$2");

        document.getElementById(obj).innerText=st[0]+"."+st[1];

    }else{

        while(re.test(s))s=s.replace(re,"$1,$2");

        document.getElementById(obj).innerText=s;

    }    

    }    

    }    

</script>