Eu estou a usar esta maneira, num dos meus sites:
No <form> adiciona: onSubmit="return defaultagree(this)" :
<form name="123" onSubmit="return defaultagree(this)">
Na checkbox adiciona onClick="agreesubmit(this)" :
<input type="checkbox" name="terms" value="accepted" onClick="agreesubmit(this)">
No Submit adiciona disabled :
<input type="submit" value="Join!" disabled>
E depois de fechares o </form>, adiciona isto:
<script>
document.forms.registerform.terms.checked=false
</script>