<td width="30%">Do you accept Terms of Service agreement?</td>
<td width="10px" style="min-width: 10px"></td>
<input type="checkbox" name="TOS" value="Accept" onClick="EnableSubmit(this)"> I agree to Terms of Service agreement.
Reason it's not working: you're not passing your function anything. Actually, because you've not used parentheses with the function name, you've not called the function at all. Passing it this in the context of the onclick HTML event attribute means you're passing a reference to the element's DOM object, giving you access to its checked property.