<script>

var checkobj

function agreesubmit(el)
{
  checkobj=el
  if (document.all||document.getElementById)
  {
     for (i=0;i<checkobj.form.length;i++)
     { //hunt down submit button
        var tempobj=checkobj.form.elements[i]
        if(tempobj.type.toLowerCase()=="submit")
           tempobj.disabled=!checkobj.checked
     }
  }
}

function defaultagree(el)
{
  if (!document.all&&!document.getElementById)
  {
     if (window.checkobj&&checkobj.checked)
        return true
     else
     {
        alert("Please read/accept terms to submit form")
        return false
     }
  }
}

</script>

<form name="agreeform" onSubmit="return defaultagree(this)">
  <br>Qui va il resto del form, comprese le condizioni...<br><br>
  <input name="agreecheck" type="checkbox" onClick="agreesubmit(this)">
  <b>Ho letto ed accetto le condizioni</b><br>
  <input type="Submit" value="Submit!" disabled>
</form>

<script>
  document.forms.agreeform.agreecheck.checked=false
</script>