function deshabilita(form)
{
    if ((form.usuario.value = "") && (form.pass.value = ""))
    { form.Entrar.disabled = false; }

    else {
    form.Entrar.disabled = false; }
}


<!--
function MM_setTextOfTextfield(objId,x,newText) { //v9.0
  with (document){ if (getElementById){
    var obj = getElementById(objId);} if (obj) obj.value = newText;
  }
}
//-->