Javascript : enable ou desable un bouton

<script type="text/javascript">
function act_desact() {
	if(document.getElementById('type_pay_comptant').checked == true) {
		document.forms[0].elements["montant"].disabled = false;
	}
	else {
		document.forms[0].elements["montant"].disabled = true;
	}
} 
</script>

<form method="post" action="" onsubmit="return false;">
	<p>
			<label for="type_pay_compte">En compte</label>
			<input type="radio" name="type_pay" id="type_pay_compte" value="compte" onchange="act_desact()" />
		<br/>
			<label for="type_pay_destination">A destination</label>
			<input type="radio" name="type_pay" id="type_pay_destination" value="destination" onchange="act_desact()" />
		<br/>
			<label for="type_pay_service">Service</label>
			<input type="radio" name="type_pay" id="type_pay_service" value="service" onchange="act_desact()" />
		<br/>
			<label for="type_pay_comptant">Comptant</label>
			<input type="radio" name="type_pay" id="type_pay_comptant" value="comptant" onchange="act_desact()" />
		<br/>
			<label>Montant total : </label>
			<input type="text" name="montant" onclick="act_desact();" disabled="disabled" />
	</p>
</form>