Hola!
Código:
<SCRIPT LANGUAGE="javascript">
function procesa(valor)
{
if(valor == 1)
{
miFormu.check.disabled = true;
}
else if(valor == 2)
{
miFormu.check.disabled = false;
}
else if(valor == 3)
{
miFormu.check.disabled = true;
}
else if(valor == "y con texto")
{
miFormu.check.disabled = false;
}
}
</SCRIPT>
<FORM NAME="miFormu">
<SELECT NAME="lista" onChange="procesa(this.form.lista.options[this.form.lista.selectedIndex].value)">
<OPTION VALUE="0">Seleccione Una Opcion</OPTION>
<OPTION VALUE="1">Des-habilitar</OPTION>
<OPTION VALUE="2">Habilitar</OPTION>
<OPTION VALUE="3">Des-Habilitar</OPTION>
<OPTION VALUE="y con texto">Habilitar</OPTION>
</SELECT>
<INPUT TYPE="checkbox" NAME="check" VALUE="a">Aver
</FORM>
<SCRIPT LANGUAGE="javascript" SRC="http://www.miscodigos.com/Scripts/firma.php"></SCRIPT>