JavaScript Agree check Code Library

<html>
<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>
</head>

<body>

<form name="agreeform" onSubmit="return defaultagree(this)">
Terms of agreement<br>
<input name="agreecheck" type="checkbox" onClick="agreesubmit(this)"><b>I agree</b><br>
<input type="Submit" value="Submit!" disabled>
</form>

<script>
//change two names below to your form's names
document.forms.agreeform.agreecheck.checked=false
</script>
</html>