<script language="javascript" type="text⁄javascript">
function vacio(q) {
        for ( i = 0; i < q.length; i++ ) {
                if ( q.charAt(i) != " " ) {
                        return true
                }
        }
        return false
}
function valida(F) {
        if( vacio(F.campo.value) == false ) {
                alert("AQUI O QUE DIZ CUANDO NAO ESCREVE NADA.")
                return false
        } else {
                alert("OK")
                return true
        }
}
function numo(evt)
{
var nav4 = window.Event ? true : false;
var key = nav4 ? evt.which : evt.keyCode;
return (key <= 13 || (key >= 48 && key <= 57) || key == 44);
}
<⁄script>

<p><FORM NAME="formulario" METHOD="get" ACTION="" onkeypress"return numo(event);" onSubmit="return valida(this);">ESCREVE:<BR><INPUT TYPE="text" NAME="campo" size="8" spry><BR><⁄FORM><⁄p>