PRIMEIRA PARTE EM JAVASCRIPT
<script>
addEvent = function(o, e, f, s) {
var r = o[r = "_" + (e = "on" + e)] = o[r] || (o[e] ? [[o[e], o]] : []), a, c, d;
r[r.length] = [f, s || o], o[e] = function(e){
try{
(e = e || event).preventDefault || (e.preventDefault = function(){e.returnValue = false;});
e.stopPropagation || (e.stopPropagation = function(){e.cancelBubble = true;});
e.target || (e.target = e.srcElement || null);
e.key = (e.which + 1 || e.keyCode + 1) - 1 || 0;
}catch(f){}
for(d = 1, f = r.length; f; r[--f] && (a = r[f][0], o = r[f][1], a.call ? c = a.call(o, e) : (o._ = a, c = o._(e), o._
= null), d &= c !== false));
return e = null, !!d;
}
};
removeEvent = function(o, e, f, s){
for(var i = (e = o["_on" + e] || []).length; i;)
if(e[--i] && e[i][0] == f && (s || o) == e[i][1])
return delete e[i];
return false;
};
function formataMoeda(o, n, dig, dec){
o.c = !isNaN(n) ? Math.abs(n) : 2;
o.dec = typeof dec != "string" ? "," : dec, o.dig = typeof dig != "string" ? "." : dig;
addEvent(o, "keypress", function(e){
if(e.key > 47 && e.key < 58){
var o, s, l = (s = ((o = this).value.replace(/^0+/g, "") + String.fromCharCode(e.key)).replace(/\D/g, "")).length,
n;
if(o.maxLength + 1 && l >= o.maxLength) return false;
l <= (n = o.c) && (s = new Array(n - l + 2).join("0") + s);
for(var i = (l = (s = s.split("")).length) - n; (i -= 3) > 0; s[i - 1] += o.dig);
n && n < l && (s[l - ++n] += o.dec);
o.value = s.join("");
}
e.key > 30 && e.preventDefault();
});
}
</script> PARTE EM HTML
<form name="form" method="post" action="">
<input name="campo1" type="text" id="campo1" value="">
</form>SEGUNDA PARTE EM JAVASCRIPT
<script type="text/javascript">
// referente ao input campo1
formataMoeda(document.form.campo1, 2);
</script>