Enable submit button problem

  
<input type="checkbox" onClick="document.all['mysubmit'].disabled =(document.all['mysubmit'].disabled)? false : true">
<input type="submit" id="mysubmit" value="submit" disabled>
 

<form name="form1" id="form1">
<input type="checkbox" name="checkbox" id="chk" onChange="chkCount();" /><br />
<input type="checkbox" name="checkbox" id="chk" onChange="chkCount();" /><br />
<input type="checkbox" name="checkbox" id="chk" onChange="chkCount();" /><br />
<input type="submit" name="submit" id="submit" disabled="disabled" />
</form>
<script language="javascript" type="text/javascript">
function chkCount()
{
// assign a 0 value to the number of selected checkboxes variable
var chkTotal = 0;
// loop though the checkboxes and count how many are checked
for(counter=0;counter<document.form1.checkbox.length;counter++)
{
if(document.form1.checkbox[counter].checked)
{
chkTotal = chkTotal + 1;
}
}
// if at least 1 checkbox is checked, activate the submit button
if (chkTotal > 0)
{
document.form1.submit.disabled=false;
}
else
// leave the submit button disabled
{
document.form1.submit.disabled=true;
}
}
</script>