function Limpar(valor, validos) {
// retira caracteres invalidos da string
var result = "";
var aux;
for (var i=0; i < valor.length; i++) {
aux = validos.indexOf(valor.substring(i, i+1));
if (aux>=0) {
result += aux;
}
}
return result;
}

//Formata número tipo moeda usando o evento onKeyDown

function Formata(campo,tammax,teclapres,decimal) {
var tecla = teclapres.keyCode;
vr = Limpar(campo.value,"0123456789");
tam = vr.length;
dec=decimal

if (tam < tammax && tecla != 8){ tam = vr.length + 1 ; }

if (tecla == 8 )
{ tam = tam - 1 ; }

if ( tecla == 8 || tecla >= 48 && tecla <= 57 || tecla >= 96 && tecla <= 105 )
{

if ( tam <= dec )
{ campo.value = vr ; }

if ( (tam > dec) && (tam <= 5) ){
campo.value = vr.substr( 0, tam - 2 ) + "," + vr.substr( tam - dec, tam ) ; }
if ( (tam >= 6) && (tam <= 8) ){
campo.value = vr.substr( 0, tam - 5 ) + "." + vr.substr( tam - 5, 3 ) + "," + vr.substr( tam - dec, tam ) ; 
}
if ( (tam >= 9) && (tam <= 11) ){
campo.value = vr.substr( 0, tam - 8 ) + "." + vr.substr( tam - 8, 3 ) + "." + vr.substr( tam - 5, 3 ) + "," + vr.substr( tam - dec, tam ) ; }
if ( (tam >= 12) && (tam <= 14) ){
campo.value = vr.substr( 0, tam - 11 ) + "." + vr.substr( tam - 11, 3 ) + "." + vr.substr( tam - 8, 3 ) + "." + vr.substr( tam - 5, 3 ) + "," + vr.substr( tam - dec, tam ) ; }
if ( (tam >= 15) && (tam <= 17) ){
campo.value = vr.substr( 0, tam - 14 ) + "." + vr.substr( tam - 14, 3 ) + "." + vr.substr( tam - 11, 3 ) + "." + vr.substr( tam - 8, 3 ) + "." + vr.substr( tam - 5, 3 ) + "," + vr.substr( tam - 2, tam ) ;}
} 

}

texto1: <input type="text" name="texto" size="20" onKeydown="Formata(this,20,event,2)">

function formataMoeda(objTextBox,SeparadorMilesimo, SeparadorDecimal,e){
    var sep = 0;
    var key = ”;
    var i = j = 0;
    var len = len2 = 0;
    var strCheck = ‘0123456789′;
    var aux = aux2 = ”;
    var whichCode = (window.Event) ? e.which : e.keyCode;
    // 13=enter, 8=backspace as demais retornam 0(zero)
    // whichCode==0 faz com que seja possivel usar todas as teclas como del, setas, etc
    if ((whichCode == 13) || (whichCode == 0) || (whichCode == 8))
    return true;
    key = String.fromCharCode(whichCode); // Valor para o código da Chave     if (strCheck.indexOf(key) == -1)
   	return false; // Chave inválida
    len = objTextBox.value.length;
    for(i = 0; i < len; i++)
        if ((objTextBox.value.charAt(i) != ‘0′) && (objTextBox.value.charAt(i) != SeparadorDecimal))
       	break;
   aux = ”;
    for(; i  2) {
        aux2 = ”;
        for (j = 0, i = len - 3; i >= 0; i–) {
            if (j == 3) {
                aux2 += SeparadorMilesimo;
                j = 0;
            }
           aux2 += aux.charAt(i);
            j++;
        }
        objTextBox.value = ”;
        len2 = aux2.length;
        for (i = len2 - 1; i >= 0; i–)
        	objTextBox.value += aux2.charAt(i);
        objTextBox.value += SeparadorDecimal + aux.substr(len - 2, len);
    }
    return false;
}

onKeyPress=”return(formataMoeda(this,’.',’,',event))”