cheka esto mas sencillito
<script language="Javascript">
/*-- for(unKnown){
[email protected] } <--do n't remove*/
function Validar(form)
{
if (isNaN(form.pregunta1.value))
{ alert("Porfa Solo numeros"); ; return; }
}
</script>
<form action="null">
<input name="pregunta1" type="text" value="only numeros"><br>
<input type="button" value="Enviar" onClick="Validar(this.form)">
</form>