<form name="myform"> 
<input type="checkbox" name="box" id="box" value="1" onclick="check()" > 
<input type="text" name="aa" id="text" disabled="disabled"> 
</form>

<script language="javascript"> 
function check() 
{ 
if(myform.box.checked){ 
document.getElementById("text").disabled=false; 
} else{ 
document.getElementById("text").disabled=true; 

} 
}