Um botão habilita e outro desabilita, setando .disabled = true;

<html>
<head>
<script type="text/javascript">
function toogle_disabled( bool )
{
	var input = document.getElementsByTagName('input');
	var textarea = document.getElementsByTagName('textarea');

	for( var i=0; i<=(input.length-1); i++ )
	{
		if( input[i].type!='button' ) 
			input[i].disabled = bool;
	}
	for( var i=0; i<=(textarea.length-1); i++ )
	{
		textareat[i].disabled = bool;
	}
}
</script>
</head>
<body>
	<form>
		<input type="button" onclick="toogle_disabled( false )" value="Habilitar" />
		<input type="button" onclick="toogle_disabled( true )" value="Desabilitar" />

		<br /><br />        
		Nome: <input type="text" name="nome" />
		Local: <input type="text" name="local" />
		<br>
		Nome: <input type="text" name="nome2" />
		Local: <input type="text" name="local2" />

	</form>
</body>
</html>