Usa OnClick o OnKeyDown ...
codice:
<HTML>
<HEAD>
    <TITLE>Untitled</TITLE>
</HEAD>

<BODY>
<form name="questionario1" id="questionario1">
<input name="tasto1" type="button" value="aaa">
<input name="Aum" type="checkbox" id="Aum" onClick="disabilita()"> 
</form>

<script language="javascript">
function disabilita()
{
var modulo = document.questionario1; 
if (modulo.Aum.checked)
 {
  modulo.tasto1.disabled = false;
 }
else 
 {
  modulo.tasto1.disabled = true;
 }
} 
</script>

</BODY>
</HTML>