Disable Checkbox Unless One Check Is Checked

Code JavaScript:
<script language=javascript>
function apply()
{
document.frm.sub.disabled=true;

[Code].....

Im using the above code. if, change this code for array of checks box not working. If i change above html code like below , Then how to change javascript?

Code HTML4Strict:
<form name="frm">
<input type="checkbox" name="chk[]" onclick="apply()" />
<input type="checkbox" name="chk[]" onclick="apply()" />

[Code]....