ti posto un esempio semplice poi ci ragioni su
ciao
<html>
<head>
<title>Untitled Page</title>
<script language="javascript" type="text/javascript">
function ResetText1() {
// alert(Chk1.value);
if (Chk1.checked)
Text1.value="";
}
</script>
</head>
<body>




<input id="Chk1" type="checkbox" onclick="ResetText1()" /></p>



<input id="Text1" type="text" value="" /></p>

</body>
</html>